CGT--electrical buzzing noise, left front speaker

[Jonathan Monetti]

Patient is 87.5 Coupe GT, digital dash 
 
Symptom is a staccato, electronic buzzing noise from left front speaker when
engine is running.  Does not vary with engine speed. 
 
Apparent cause:  A few weeks back, was cleaning up crud and tarnish on
underhood electrical connections, including using dielectric grease.  I
broke the fuel pump relay trying to get it out.  It still worked but the
buzzing described above started.   I ordered a new FPR, popped it in and the
speaker noise went away.  Now the buzzing is mysteriously back (the car has
not been driven or worked on since).   
 
I am sure this will end up being a gremlin hunt, but maybe someone knows:
in a 4k/CGT, is there a 'typical' cause of an electrical buzzing from just
the left front speaker?  I am thinking a random smear of dielectric grease
could cause it, but I can't fiugure out why it's only on the left side and
how it could go away with the new FPR and then come back on its own.
 
My dark fear is mice eating at wire insulation, but there is no scent or
mouse dirt anywhere in the car, and it's been surrounded with repellent
since late fall. 
 
TIA
Jonathan Monetti
'02 A4q 1.8t
'87.5 Coupe GT