Faint brake engagement when driving...

[Qube]

Trying to narrow down an issue...

Symptom:
- [assumption] occurs only driving over 50kph
- gentle brake engagement intermittently (every 5-10 seconds for 1-2 seconds)
- only occurs on throttle, not when coasting
- appears to happen quicker between engagements at higher speed (eg 100kph)
- not enough braking to stop but enough to feel and notice a very slight drop in revs/speed -- very, very slight
- seems like only passenger front by sound and lots more brake dust

Tried:
- caliper rebuild and disc refinish

There are no lights, no codes, nothing other than the faint grinding sound every so often. It's "on / off" grind/sanding sound without any apparent fade.

It APPEARS to have a pattern when driving at a constant speed on throttle (eg. 2 sec on, 4 sec off, repeat) but unquantified.

Car is a 2002 540i so it has DSC. Again, no lights are on or turn on when that happens. Also happens if DSC is turned off (just for the heck of it).

I want to try and see if the sound timing is exactly the same every time at a certain speed, but beyond that, I'm stumped.
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[Tiger]

I would clean the sensors related to traction control/abs. If this doesn't work then the easiest and probably the cost effective repair would be a trip to the BMW dealer who has the proper computer diagnostic equipment to detect the culprit.

How about your tires... are they all the same brand? similar wear? Not one brand new tire and other three old? This might trigger the DSC.

[Qube]

Only happens on a consistent application of throttle like cruising at 70kph. Don't notice on accel or just coasting.

Buddy had a good idea of hooking up a reader and logging the output of each speed sensor. Hopefully the pass right will show an inconsistency.

[Tiger]

Something had to be triggering the DSC... maybe faulty brake switch.

[Qube]

Well some more revelation today:

1. appears to only happen over 50kph
2. nearly always happen during gradual acceleration between 70-80kph
3. happens intermittently above 80kph even at constant throttle
4. never happens when coasting

Had the car up on a lift and can replicate the same sound by accelerating in any gear (easiest to hear in 4 or 5) to about 80kph then SIMULTANEOUSLY tapping the brakes.

The sound is isolated to the LEFT (driver) rear. The axle shaft on that side is looser and has a lot more lateral play than the right. Upon the "move", it appears the shaft moves outwards (as it should) and the sound occurs ALONG with a very small drop is speed/rev. This may be due to the inherent friction slowing that drive wheel down (?)

A rebuild of the axle and replacement hubs are on the list.

Having said that, would this cause added brake dust on the RIGHT front? That is, the complete opposite end of the car? Could it interpret that speed difference there and actuate the opposite front brake? There is definitely increased brake dust on that side which may or may not be a symptom.

Having a data logger and watching the wheel speed through Autologic (cannot monitor ABS when driving), there doesn't appear to be any change in speed per wheel. Not noticeable at the resolution and sample rate it's running any way.