Alternator Noise

I know that this is another frequent topic, but I'm hoping that someone
can help narrow this down for me.

I have a rebuilt Chrysler alternator (from Electrosystems) on my
Cherokee that was installed about 400 hours ago. However, I am
experiencing alternator whine that varies with engine speed, but which
disappears completely when I turn off the alternator side of the master
switch. In addition, increasing electrical load seems to make it worse.
I have replaced the filter capacitor on the alternator output with a new
one (15uF) and it doesn't seem to have made much of a difference (the
filter is grounded to the alternator).

My mechanic took a look at the alternator at the last annual inspection
and thought that the cooling fins were backwards (auto style, not
aviation style). Not sure if that's normal for the Cherokees or not,
but I can't believe that Electrosystems would do it wrong.

How long should I expect this alternator to last? Does this sound like
a rectifier problem that would warrant replacement of the alternator
after just 400 hours? Could it be a regulator problem? Any suggestions?



Thanks,
JKG