87 Prelude 1.8 Carb Alternator Cont'd

So I have replaced the alternator on the Prelude, (215000Kms)very easy once
I removed the motor
mount and raised the motor up a few inches, even
easier than my 98 civic.

Anyways, I noticed during the test drive that
there is a very faint *pop* noise when I hit the
accelerator pedal. The noise is coming from the
speakers! When the stereo is off, there is no
noise.

NOTE: I replaced the brushes and bearings on the
alternator. I then brought it in to get it
tested at the local autoparts store, and it
passed. Not a big fan of these testing machines
as it gives no relevant data, just a PASS or a
FAIL.

Also, It appears that there is a filter on the
back of the alternator, I left it in place.

Any ideas? should I be concerned?

t