I have always been intrigued by unusual brake noises.
Recently we discovered an odd brake noise after replacing rear brake pads on a 2007 BMW X5. The customer came to us after noticing a warning light for the rear brake pad lining, and we performed the rear brake job with new pads, new rotors and a brake pad sensor.
Once that job was done, we still heard brake noise – but not from the rear.
After replacing the rear brake pads and rotors, the front brakes started to make noise. It did not start right away, only after the test-drive while we were bedding in the new rear brake pads and rotors. It started out as a light squeak but soon sounded like a fire truck coming down the street. The other interesting thing was that the noise was only coming from the right front wheel.
One could typically assume that the front brakes were also worn, yet they were replaced by another shop a year earlier and still had 9 mm left on the pads. We then rechecked the front brake pads to make sure they were wearing equally. All four front brake pads were equal, with no abnormal distinctions, and the front rotors did not have any hot spots or heavy grooving.
We pulled out each of the front brake pads to make sure they were not sticking in the caliper carrier; all four were sitting in the caliper correctly.
Finding a solution
We often see brake pads that were not prepared properly for installation when new. So, we prepared each front brake pad by applying a high-temperature brake grease and properly cleaned the caliper carriers. We then reinstalled the front brakes and test-drove the car again. The noise was still there.
After a brake repair, there is a change in brake pressure from the brake master cylinder and brake calipers. As a brake pad wears out, it gets smaller, and that distance has to be made up by the brake caliper piston.
After replacing the rear brake pads, the rear caliper pistons have to be pushed in to compensate for the new brake pad. In turn, all four brake calipers will exert a little more pressure to the brake pads, as there is less distance for them to travel.
This added pressure could be the reason for the noise. Small enough as it may be, this alters the way the brake pad addresses the rotor. Subsequently, this causes a high-frequency vibration and then noise.
After reviewing all of this information, my technician and I asked ourselves if there was something we were missing. The technician said he thought the lock clips for the front brake pads could have felt a little tighter.
On some cars, the brake pad hardware comes with the pads; on this BMW, the lock clips for the front brake pads are sold separately. We then called the customer and got the OK to replace the lock clips.
After the next test-drive, the noise was gone.
I made sure to take the car on several test-drives before returning it to the customer. Only one time did I hear a faint brake noise, but it was nothing like the loud sound – similar to a fire engine coming down the street – that we heard before.
As I wrote earlier, it is not unusual to have brakes make different sounds after a brake job, but this one really tested me.