I have an 99 Subaru Imprezza and also get alternator whine.
After the thrid visit back to the fitter they came to the
conclusion that the problem is with the alternator itself.

If you actually listen to the alternator you can hear it.
And if you remove the connector to that alternator that
recharges the battery(?) the whine goes.

I've still got to talk to my Subaru dealer about the possibility of getting this replaced under warranty.

Then again it may only be a line the fitter made up to get me rid of me! I'd be intrested if this may be Subaru releated.

As to the background noise I do get this to, certainly in the car, but only when the engine is running - when the engine is off and the empeg is running purely from battery its pretty quiet. But then again the gain on my amp is currently (no pun either!) turned right down to reduce the whine!

Regards

Paul