Humming sound from rear brakes?? (95 E420)

I have noticed that when travelling at speeds of 70-90 mph, after maintaining speed for about 10 seconds, a faint humming sound begins which increases to a very audible level (easily noticable above wind noise, a/c, etc). When the brakes are applied, the noise stops. One MB tech said that the emergency brake shoes could be breaking down, and in turn create a harmonic resonance from the particles. Any ideas????

[stevebfl]

Sounds like the pads aren't getting far enough from the discs after application. This usually happens after replacement where the pads are exactly flat and the disc has dished wear with a certain amount of lip.

Depending on what has been done the answer (to your problem) changes. New discs and pads will probably solve it. New pads of a softer compound will also probably do the trick.

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Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician

[andy day]

I would tend to agree with Steve, bear in mind that the bulk of the brake work is done by the front set and if a new set of pads and discs ( rotors ? ) have been fitted recently, it may take a while for them to bed in. You might like to carefully file a champher on the leading edges of all the pads and this will take off the sharp edge of the pad where it contacts the disc. This is often necessary with the harder pad that takes it time to bed in. I would look at the emergency ( hand ) brkae after looking at the main pads and discs as they do very little work indeed, other than to keep the car form moving once stopped. Hope this helps

Thanks for the info guys. I replaced the front pads about 7k mi ago. The rears I have yet to replace. I will replace them and see what happens. Like I said, it stops after I lightly apply the brakes or make a sudden lateral move, but always starts back again. I use my parking brake daily since my driveway is on a hill..... any thoughts on the e-brake being the culprit??

[mbdoc]

The parking brakes are shoes inside of the rear disc, doubt that touching brake pedal would have any effect on them.

Hi Trapper;
I have a 1989 300SE that has a low humming sound while it is being driven.It doesn't matter how fast you go,the sound pitch stays the same.A hard right turn makes it stop and a hard left turn makes it a little louder.As soon as I touch the brakes,it stops.I have had it in the shop twice and they could not duplicate the sound.I know this doesn't help you with your' problem but if I ever resolve it,i'll post back.Good luck.