Today finally I had time to go back to the hangar and attack the noise makers in my plane. Concentrated on a little cooling fan, since it was easier to work around.
Installed a relay activated by the same switch on the IP. That reduced the length of the power and ground feeds to few inches. Very little difference.
Removed all the feeds to the fan and powered it with external battery. No noise.
I had enough for today, plus I'm out of ideas. So I went for another sunset flight over the beach with the wife in a rented C-172.
Tomorrow my electronics engineer friend is coming with his oscilloscope. I also have a reel of shielded wire to attack the problem on different front.
New ICOM radio is on it's way.
That's all for now.

Bulent "Buly" Aliev
Making time in C-172 instead of the CZ
Ser# 066 / N484BD
http://tinyurl.com/dcy36