ground loops. ug.

I have a ton of alternator whine in my system. If I remove the alt belt there is no noise, if I remove the field wire from the alt there is no noise. and needless to say, without the engine running there is no noise.

what I've done so far is to use one power lead and one ground lead, both with inline filters, direct to the battery for the empeg and the amp. If I remove the RCA inputs to the amp there is no noise.

I tried adding a ground line to the empeg sled, tried grounding and isolating the amp case.

odd thing is that from time to time, while driving, the noise will fade away for a short spell and return. this may be that the alternator is failing, but that is a long shot.

the car has a new battery, a new voltage regulator (both changed to cure problem).

a scope test shows a normal alternator pattern.

I'm stumped.
can a ground loop be 'seen' (voltage drop, scope pattern.....)?

i read through the FAQ, checked the crutchfield help site.
odd thing is that as i turn the gain down on the amp, the noise gets worse.