Having (as a dealer) been in discussion with ICOM, who are aware of the problem and very helpful on this, the story so far:
The problem is RF feedback into the mic amplifier causing 'howl around' on transmit. The mic input is not adequately filtered.
The effect is very headset dependant, with David Clarks the worst affected of those I have tried. The cheapest headset tried (Flightcom 4DX) was least affected, with assorted Sennheisers usable but an annoying background buzz.
ICOM have already produced a modification to the mic circuit for a different problem, which improves the behaviour, but doesn't cure it. ICOM UK are still working on a fix with the worst case setup we could find.
Update when available........