radio squeal question.

radio is an icom a200
headset is a clone of a dave clark, an australian altronics C9073

when taxying and early into the flight when I press the ptt the radio
squeals in the headset.
after a while in the air the squeal vanishes and the radio setup works
just fine.

I changed headsets today while taxying out and discovered that the
squeal vanished with the headset. The new/different headset worked
fine.

I thought it might have been moisture/condensation and have wd40'd the
connectors without changing the problem one iota.

anyone an idea as to what would cause this problem?
all equipment appears to be in sound working order with no damage.

Stealth Pilot