Radio sidetone problem

Hey again.

I have a new TKM Nav/Com (Michel MX385) in a 1982 C152. The radio is great,
clean, lots of volume. The sidetone, however, is very faint, almost a
whisper, and it's making me nuts. On a rare occasion, while I'm
transmitting, the whisper blasts out for a brief second, and returns to
nearly imperceptible. The intercom is about 5 years old, but the box over
the radio seems to be original (Nav 1/2 switch, sidetone VOL). It has a
sidetone 'volume' button which rotates freely, and seems to be turning a Pot
inside. Has anyone had this issue, and what is/was the resolve? I don't want
to pay a shop 4 hours for an easy fix issue.

Thanks in advance.
Marty