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Question DetailsAsked on 12/4/2013

In zip code 21114, which is the most reliable auto repair shop in the area? I HEAR KNOCKING SOUND WHILE BRAKING.

My car is a 2004 Pontiac Sunfire with about 129,000 on it. The front struts, shocks, stabilizer bushing has been replaced and 4 new tires were installed last 3 months ago by Pep Boys, Annapolis. At first the knocking sound ceased but after sometime it came back. The sound is prominent while gently stepping on the brakes, but surprisingly, when the weather is cold or the temperature is below 40 degrees, the knocking sound ceases. I brought the car back to Pep Boys 3 weeks ago and also to West Side Repair Shop in Crofton, MD but both shops found no problem with the car. The Pep Boys Auto Repair Manager told me that they cannot do anything more to solve the knocking noise, while West Side cannot replicate the problem. Also, 4 wheel alignment was done by AllTune of Severna Park, MD, last week but it did not solved the problem.

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My recommendation is to find a reliable professional auto shop, with an ASE Auto and Light Truck certified mechanic with suspension/steering AND brake certificate with tests A4 and A5 (suspension/steering, and brakes) working on your problem. The "mechanics" you find at places like tire shops, muffler shops, oil change shops, and chains like Pep Boys and Sears and such tend to be the less qualified and less experienced - many ofthem just kids fresh out of high school or still in high school with little or no professional repair training, and for your issue you need an EXPERIENCED front end and brake mechanic, either a reputable long-time full-capability repair shop or a GM dealer with good reviews and recommendations.

There are a TON of things that can cause this sound, mostly initiated by the vibration caused by normal slightly out of flat front discs or a high spot on a rotor, which makes for a slight wobble in the front end so anything loose can move and knock as the wheels go around - which can cause many types of loose things to rattle in sync with the rotation of the tire, even if the item rattling has nothing to do with suspension or wheels.

Possible causes I can think of, not knowing exactly what it sounds like:

lug nut or broken lub bolt rattling around in hub cap, missing wheel center hub spacer ring - some cars have different size bushings on hub to allow use of different size wheel center holes, missing or loose lug nuts allowing wheel to wobble, wheel installed not flush and tight to hub so it is slightly off kilter - especially common with vehicles with tight-fitting center hubs like Dodges with aluminum wheels where it may feel seated when you put it on but is actually sitting at the face of the center hub ring and has 1/8"-1/4" further in to got before tight to mounting face, wheel lug nut holes worn or wormed out (usually occurs with aluminum wheels), directional brake pads installed facing wrong way so not fitting in calipers right, worn or loose wheel bearing, worn driveline or axle U joint or CV joint (front wheel or 4 wheel drive vehicles only), loose or worn axle or differential, brake piston jamming or cylinder corroded so piston is not moving all the way out when brake is put on - leaving room for brake pad to vibrate, worn suspension, damaged or missing seal in shock absorber so outer shock casing rattles against inner shaft, loose metal backing plate on brake pad (rivets holding pad wear surface not riveted tight so brake pad is not pulled tight to metal plate), rebuilt brake pad backing plate used which was worn so does not fit tight in caliper or was ground to remove rust making it thinner than normal, loose or worn tie rod end, worn rack or pinion in steering, worn steering gear U joint, worn ball joint, worn or corroded or stripped or loose brake caliper mounting bolt or pivot pin, brake pads sticking in caliper or calipers sticking because not lubricated, loose shock absorber bolts, loose or broken axle mounting bolts (to frame), deteriorated or missing axle mounting bracket rubber pad (for those cars with one), brake pad not properly seated in caliper, caliper worn/corroded so brake pad backing plate fits loosely, missing or broken brake anti-rattle spring, moisture or air in brake fluid causing piston and pads to oscillate during braking, defective ABS sensor causing ABS system to kick in before it should, brake line or mounting bracket too close to wheel so being hit by wheel or rotor as it turns, something in engine compartment rattling like a loose radiator or body panel or bumper or headlight housing or washer fluid tank, PCV valve or brake master cylinder chattering because of of air leak in brake vacuum line (this happened once to me - drove me half crazy locating it - took about 3 hours to tie it down to that, and only after hooking a pressure gage to the brake line to check for fluctuations).

Obviously cannot diagnose effectively over the web, but the light pressure braking knocking and low temperature no-knock leads me to think initially that it is some lubricated part that is loose, but which during hard braking is held tightly in one position so it does not knock, and when cold the lubricant keeps it from wobbling back and forth. Try to remember (or test) if it does not do it at all below 40 or so, or only below 40 when you have not been driving it - cold grease in wheel bearings or cold brake pad silicone lubricant could conceal a sound that you would hear when grease is warm (either from warmer tempeature or from having driven 10 or miles or so) and lets the part rattle. My first guesses would be a caliper fitting or mounting bolt issue, second a brake pad issue, third wheel bearing, fourth - well, any other rest of the above.

I think your first step should be figuring out how to replicate the problem on demand - find an long empty parking lot and try to find a condition where it will consistently do it - with different brake pressure, at different speeds, turning left or right, going forward or backward, rolling in gear and out of gear, sitting still and applying brakes, and sitting in park and both of you rocking the car up and down on corners and on nose and tail and rocking it back and forth sideways - and record under what conditions it does it. Also, use two people (one driving, one hanging out window or standing outside listening) to check where sound is coming from as you drive - front or back, left or right, and if louder when turning one way or the other or not. At least this will give you info for the mechanic so he can hear the sound reliably when you take it in.

IF you are into light mechanic work yourself or have a friend who is, crawl under car and try shaking everything you can reach in the suspension, exhaust, body, driveline and steering system to find anything loose. Then jack up one wheel at a time, and by pulling out with one hand and pushing with the other try wobbling or rocking the wheels top to bottom and front to back to feel for any loose mounting or ball joints or wheel bearings. Also check brake hose mountings and wheel wells for anything loose. While jacked up and car in neutral (with chocked wheels and emergency brake on) turn wheel forward and backward, both free rolling and with someone putting the brakes on lightly, to listen for any rattle or grind.

If you can tie it down to one wheel, take that wheel off and check all exposed parts for looseness or wobble, then with someone lightly putting brakes on check that all the pads are making contact equally - that there is not one still loose or moveable at one end when the other one is in contact with the brakes on lightly, and that the caliper (mounting frame for brakes) has no slack in it with brakes either one or off.

Good luck - this kind of thing can be frustrating - as expensive it you cannot at least partially isolate where it is coming from. Please check back in her (with the Answer Question tool) to let us know what it finally turned out to be.

Oh - one other thing - if you have kids, check for loose toys like marbles rolling around on floor boards or in trunk.

Answered 6 years ago by LCD

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