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Question DetailsAsked on 1/16/2014

If someone is draining water from second floor bathroom sink, I can hear loud dripping sound downstairs

I noticed
recently sometimes if someone is draining water from second floor bathroom sink, I can hear loud
dripping (or knocking) sound downstairs. The sound went away in 30 sec. I first thought something is vibrating downstairs, but later on I confirm the sound is from above the ceiling. I monitored the ceiling
for a couple of weeks, and haven't noticed any water leak.

Could you please shed some light on this issue? Thanks.

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5 Answers

Voted Best Answer

1) I am going to assume you do not have any openings into the ceiling to easily look for leaks under the basin. Very commonly light fixtures have large enough base plates or bases that if you remove one, you can nibble away a bit of drywall without it showing once the light is back up, and sneak a fiber optic inspection camera (usually 1/2" hole) up next to the electrical box it mounts to, to look into the floorspace above. You can rent these for about $25/day - purchase for $100 range. Be sure to caulk your hole when done if the box was insulated in.

2) I am also going to assume you have checked under the basin for leaks, and on both sides of walls and floors both there and downstairs for leaks or mold smell.

3) First thing I think of is creaking pipes - rubbing against the joists and tuds they run through as they expand and contract. This is VERY common with hydronic and steam heating system pipes, which creak and groan and click against the wood framing (or if they are installed with no slack) as they heat and cool - VERY commonly sounds like a drip, commonly initially faster and then increasing in interval between drips till stops. Usually far more pronounced with pipes carrying hot water. Unfortunately, this is also what a lot of drips do - drip moreas the water goes through, then tapers off to residual drips as the last of the water leaves the leak - especially if at a 90 or trap that holds water for awhile. If this is it, if sewer pipe (probably plastic) will do it probably far more with hot water use than cold. If water pipe, then obviously would only do it when that one side of the faucet is used. Test by running only hot or cold, and listen at the pipes under the basin to hear (or feel) if one is clicking afterward.

4) I can think of one thing more likely thing, in addition to above and those mentioned in other comments, that causes this - more commonly happens in rain gutter downspouts, but I have heard of it being chased down in sewer pipes under kitchen sinks and washing machine drain pipes. If there is hair or string or whatever in the trap which extends far enough so it acts as a wick from the trap into the drain pipe going into the wall, it can slowly drain water out of the trap down the drain pipe. If the pipe is installed so the horizontal pipe and elbow from the sink extends past the "front" edge of the drain pipe below, then a small trickle or drip of water from that pipe can free-fall into the vertical pipe below instead of flowing down the "front" side of it. This can cause a splash or drip sound when it hits bottom at the next bend, especially if that bend is a little low relative to the outgoing pipe or is a glued or female 90 (which normally has a bit of a low spot at the bend) so it retains some water all the time. Think of the pipes being sort of in a Z configuration but with the angled part of the Z almost but not quite vertical, so a drip from the top leg of the Z freely drops down the angled part, and hits the pipe or standing water at the start of the bottom leg of the Z.

Permanent solution, if this is the case, is to redo the pipes so the Z "flattens out" - so the water from the top runs down the "front" or near side of the vertical pipe instead of free falling, so it does not make noise. Rain gutters commonly have this problem, with drips falling 10-20 feet or more before hitting bottom in a metal or plastic pipe during light showers or slight snow melt - can drive people mad especially at night - like the Chinese water torture Hollywood used to love so much.

Short term solution, if it is caused by a wicking action from the trap - remove and clean out the trap, and also swab out the horizontal run (about 1-2 feet) from there into wall - just don't lose your swab in the pipe. I use an old sock several times stapled and well duct taped to a stick for the swabbing.

5) My favorite investigative tool evenn before thermal IR to find leaks - I am sure some of the other contributors will say Oh no, here it comes again - is get a $10 METAL (not plastic) stethoscope at your local pharmacy or box store pharmacy department, and after using basin, start searching - listen to water pipes, walls, floors, sewer pipes - you should pretty quickly be able to track down loction in space where the noise is coming from, which should give you a pretty good idea which pipe it can be - and if louder in ceiling or wall than pipes, then is probably a drip into the wall or ceiling, which might not be showing up because the wood or insulation is soaking it up as fast as it falls. Also, if in wall or ceiling you will hear the actual "splash" as it hits.

6) Do not let this slip, just in case it is a leak - because water damase will start, and mold and fungus will start growing - especially fast if a sewer pipe leak.

Good luck and please let up know what it turns out to be, using the Answer This Question button.

Answered 6 years ago by LCD


Are you sure it is the water draining that is causing this? If this is a recent sound it sounds to me like it is a water hammer problem. Nothing I can think of would make a kocking sound from a drain, dripping maybe. I doubt you would hear a drip though through the walls. My best guess without being there to hear it is a small hammering. Is this a two family house? If it is not or you are the owner and have access to the upstairs bath I would try openning and shutting off the water in the sink to see if this is the cause. If this is what sets it off you may have a flush valve going bad in a toilet. For some reason when they are failing they will creat a stuttering or hammering noise starting of as a less pronounced noise and getting louder as the leak gets worse. If this sets off the noise you can isolate which toilet is the problem by shutting of the water supply to each toilet one by one untill it does not happen. If this is it you might change all the flush valves at the same time because it is going to be cheaper than calling a plumber as each one fails. That is if the toilets are about the same age.


Answered 6 years ago by ContractorDon


Hello Don,

Thanks for your answer. This is SFH. The noise only happened when someone is draining bathroom sink on the second floor. Flushing the toilet, using the shower in the same bathroom has no problem.

Answered 6 years ago by shiyang100


When the stopper for the sink is pulled? Unless the bath upstairs was added later and there is a mechanical vent I can not think of anything that would make a knocking or dripping sound. Maybe some of the others that help here can come up with a different idea. It really does sound like water hammer. I guess if the sink vent is blocked it could slow down the flow enouggh to cause a pipe to vibrate as the water bubbles out. Unlikely but possible. Do you hear it when you drain the sink or is it possible someone else is draing the sink and turning off the faucet at the same time. It can happen that one faucet will do it when say the shower or othe fixture does not. Some shut the water off more abruptly than others such as those with a cartridge and they are more likely to do it.


Answered 6 years ago by ContractorDon


Hello LCD,

Thanks so much for your answer.

I did some tests at home last night. With the same sink, I was upstairs, and my wife was downstairs. I first started with running cold water. No noise. Collected enough cold water and drained it, no noise. Running hot water, the noise appeared in about 15 sec. I can hear it hehind the wall on the second floor, and my wife heard it above the ceiling. It is loud, fast, high frequent knocking/tapping sound. Turned off faucet, the frequent noise stopped immediately. But in the next 5 min, you still can hear knocking sound every 15 sec or so until it completely went away.

So based on your suggestion, it might be air in the pipe or thermal expansion. So what can I do?

Answered 6 years ago by shiyang100


Sure sounds to me like thermal expension and contraction. If that is the case, figure out if it is the drain or water pipe this way (or use stethoscope on pipe). Run full hot water into the basin - making sure the plug is holding well. IF noise occurs, then is hot water pipe. IF not, release plug to drain - noise should occur, so it is drain pipe.

Then try wadding a piece of fabric around the respective pipe where it penetrates the wall under the sink - you might be lucky and find drag point is there. If that area has been drywalled or caulked up tight, that could be your problem area - using a utility knife or hand-held coping saw blade, without cutting the pipe, open up a small gap around the pipe. Ditto whereever you can access the pipe below before it goes up into wall/floor - in basement or crawl space, maybe.

If that does not work, then problem is where it goes through a stud or joist in the wall or floor - to choices to try:

1) seen if you can wedge the pipe with something non-metallic, to pull it tight - either at basin or in basement/crawlspece - sometimes wedging it tight will stop the noice.

2) get at the place it is rubbing, which means opening up the wall to get to it, if it is that annoying. Use a METAL (plastic does not work well) stethoscope for $10-15 from a pharmacy or box store to pin down where it is making the noise, open drywall up at that spot, and either open up any tight hole it is going through, or if due to dragging on a stud or joist it is laying on, put in a rubber coated pipe hanger to hold it up off the wood. Just be careful if cutting the hole larger not to knick the pipe - I wrap the pipe in a piece of curled light gage flashing metal to protect it, drill a small starter hole next to the pipe, then cut the existing joist/stud hole larger halfway around with a coping saw plade wrapped in duct tape - using power tools is iffy in that situation, though some people jump in with a sawzall and hack away. Some people just slip in some vapor barrier plastic sheet or heavy waxed paper wrapped around the pipe, slipped through the hole in the joist or stud where it is rubbing, and stapling it to the wood on both sides. This can work in many cases, but over the course of years can wear through and start making noise again.

Whatever you do, do not close up any opening for several days at least, to be sure noise has gone away.

Answered 6 years ago by LCD

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