Help diagnose this @#$#%^@ E34 noise!

[Jonathan Schwartz <jonathan@xxxxxxxxxxxxx>]

Fellow BMW junkies:

Over the past year or two, I've been living with the most annoying 
problem: A noise that I have not yet diagnosed.  I need expert help.

Here's what I know:

WHERE: The noise is coming from the engine compartment, driver side of my e34

WHAT: When it occurs, it lasts just a second in length.  It sounds 
like a small motor revving up and then back down again, with a low 
gutteral sound.  Kind of like an air gun, but 3 octaves lower.  I can 
feel the source of the noise as well, thru the gas pedal.

WHEN:  It occurs nearly every time I start the car, back out and then 
take off.  It also occurs when a tire hits a pothole, and think that 
it is limited to the front passenger wheel.

MORE: I really think that it has something to do with the ABS system. 
My very capable mechanic doesn't think so.  He suggests that failures 
of the ABS System are rare. Plus, a light will indicate that problem. 
Plus, it is impossible for the ABS System to operate without the 
brake pedal being depressed (It happens with the brake pedal being 
depressed).

MORE: In an attempt to isolate, I pulled the fuses for ABS.  No luck. 
Cruise Control. No luck.

MORE: Did a little research online and discovered that some ABS 
systems have electric motors/pumps to help restore the high pressure 
reservoir side of the system, to provide an emergency backup in the 
event of simultaneous failure and need for the ABS system.  I 
theorize that I have a leak in the system, and it needs to recharge 
periodically.  But that doesn't explain the pothole occurence.

MORE: The same problem happens occasionally on my wife's 89 325i.

Does anyone know ABS really well, and/or know what this sound might 
be of not the ABS system?

TIA,

Jonathan
- ---------------------------- * * ----------------------------
Jonathan Schwartz,
Mill Valley CA
jonathan@domain.elided

'95 525iaT
'92 525i
'89 325

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