Thanks Ed for the info.
I shall follow up on it tomorrow and check out that connection.
The thing is, it works fine on ground no engine running.
After engine start, left side decides to cause the trouble while the right
side just plugs along as usual.
But I have to beat this as it is irritating to say the least.
I suspect it is also degrading the quality of the IC as well.

Scott - ol' GH - Thanks for the info.
I AM glad you join these ranks in which I permanently reside - you know -
brain out of gear - although in your case I dare to suggest a mere
nanosecond lapse.
Now, I have done just about all the suggestions which have come in, and the
next thing is to go over them again and make sure I haven't missed anything.
The ground Tx to the handheld fifty yards away is loud and clear. No squeal,
no feedback, nothing.
Both sides, and both radios.
The unfortunate passenger, whom you have met, hears the squeal as well when
I Tx.
I believe the wiring was originally done by electronics expert - developed
heart pacemaker - and as this is a relatively recent phenomenon, I doubt it
is an original fault. Not to say it hasn't been fiddled with by someone
unknown, especially when I was over enjoying Fredericksburg hospitality.
I wonder if it might be IC / radio Tx conflict, as the squeal does not occur
when the IC switch is OFF  - yet it is only LHS.
Anyway, the pursuit continues.
Meanwhile, I have to search for a new home for CZJ, as RAAF is kicking us
out of Point Cook, on 30 days notice, and we are not amused. They aren't
doing anything with it, just kicking us out. It is a real problem as
affordable hangar sites are like hens teeth.
Thanks for the help to all.

Dom
VH-CZJ