Question about brake noise while driving

Old 03-11-09, 09:35 AM
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Question about brake noise while driving

Posted this in the wrong section earlier.....sorry! I'm hoping to confirm what a person told me is causing a brake related noise before doing the repair, and I'll try to keep this short as possible. I have an '06 GMC Sierra extended cab, 1/2 ton, 2 wheel drive,with only 13,000 miles on it. And I'm a contractor, and a couple weeks ago, I was coming back from a job that was a ways up a mountain road, and coming back down this road is not the best on brakes, especially when a truck as high geared as mine isn't easy to slow down by downshifting. Anyway, I've been on roads like this here and there, so I'm sure my brakes have been getting glazed over time. I get to the bottom of this hill and I'm getting a high pitched squealing noise that I know is glazed brakes. That wasn't a big concern, but I noticed when I got on flat ground I was getting a "chirp" that was keeping with the revolution of the wheels while driving, no braked being applied. I confirmed it's coming from the left front wheel. It's the same pitch as the glazed brake sound, but like a said, more of a "chirp". I get home and figure I'll see what happens the next day. In the morning, it won't do it all.....all the way up to this unusual noise, but I'm also not braking nearly the same way. Anyway, coming home, same thing......squealing brakes, and the chirp is back on flat ground. To fast forward, I end uo going to my mechanic, and ask him what the chirping when I'm driving is coming from. He says it could be a lot of things, but he'll check it out if I want. He takes the front brakes apart, scuffs the rotors, scuffs the pads(which were glazed good), makes sure everythiing is as it should be, adjusts the back brakes, and puts it all back together.He even commented that the pads looked like they had got pretty darn hot, and the rotors had a nice mirror like finish. My squealing brakes are gone, but after I drive a bit and apply the brakes a few times, and not necessarily going downhill, the chirp is still there. I had to go to another out of town job the other day, and go over a different mountain with not nearly the downhill slope, but you still have to brake a fair amount. Anyway at the bottom, I turn onto a highway, and taking off from a stop, there is the chirp, and it speeds up as the truck speeds up, but I noticed it goes away if you keep driving, and don't apply the brakes. Further experimentation proves that this happens after you have braked at least few times, so it is definitely related to the brakes/rotors warming up, and does go away if allowed to cool off a bit before braking. Now, my mechanic says the next step would be to turn the rotors and put new pads on, and that maybe the rotor is just not perfectly true, and is just barely getting touched by the pad as the wheel turns. Let me also add that there is no pulsating brake pedal, indicating a warped rotor, so that isn't it. Anyway, his estimate to turn the 2 rotors and put new pads on was a little pricey, and I'm considering doing it myself, so when I called to get a price on the pads and started to describe the problem just like I did here, the guy kind of stopped me and said(with confidence) that what was happening is that I have hard spots on the rotors from them getting hot(which they have), and that those hard spots will "bulge" out, if you will when the rotor gets warm/hot from braking, and can just barely come in contact with the pads while driving, and make this high pitched "chirp". This makes a lot of sense, and he told me this like he's seen this before, and it also explians why if you drive a bit without braking, or let the truck sit, the chirp is gone......until you brake a few times or so. So my question is; does this sound like the cause of the chirping noise to anyone? He also said you could probably turn the rotors, but the hard spots are still there and the noise could come back, or not. It's a hit and miss. Sorry for the long story, but I wanted to be thorough, and if you made it this far, thanks for reading, and really appreciate any feedback. Thanks in advance!
Old 03-20-09, 05:48 PM
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RE:Question about brake noise while driving

The most likely cause of your brake noises is that the front rotor is slightly warped, proabably within allowed tolerances so you dont "feel" it when your applying the brakes but the warpage in the rotor lets the pad make and break contact with the pads causing the chirping sound you are hearing. after driving a little while this constand pushing of the pad against the rotor will cause the caliper piston to retract back into the caliper so the contact stops then when the brakes are applied it starts the whole viscious circle over again.
Most brake lathes used to turn rotors in private shops are worn and neglected enough that the runout that is actually left in the rotor after it is "turned" is usually beyond that of allowable tolerances therefore GM and several other car makers don't recomend machining the brake rotors and just say to replace them with new ones. besides that the thickness between new and discard specs is on most newer vehicles only 20-30 thousandths of an inch to begin with so when some metal is worn away with the brake usage, and then more is removed to "turn" them what you are left with is a rotor that is barely above "discard" thickness anyway, and then when used severly as most people do with stop and go traffic and descending hills with the higher geared over drive transmissions in most modern cars the brake rotors will just warp again rather quickly because they no longer have enough mass left to absorb the heat and then cool slowly to avoid warpage. besides most rotors at a do it yourself check or autozone type parts store are around $40 so them and a set of QUALITY pads shouldn't be but around $100-$120 bucks. not that much considering what is at stake if they don't work like they should. another trick to keep brakes quiet and avoid squeeking I have discovered is to put some silicone grease ( like the dielectric grease you can get for electrical systems) between the caliper and brake pads where they make contact with each other. Don't use petroleum grease as this will ruin any rubber parts it makes contact with and never use any lubes between the brake pad and rotor surfaces.

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