Ok here are the symptoms on my 82 M20k
1. Alternating medium and high pulsating noise in intercom headsets started after avionics shop installed JPI 900 and removed GEM 601
2. Radios in the panel: GNS 430w, GMA340 audio panel, kx165, strike finder, 696 in doc
3. Noise occurs at all four seats in different brands of headsets
4. Noise does not occur on speaker
5. Noise occurs with engine off
6. We sequentially pulled each breaker and noise still occurs regardless of any individual or combination of pulled breakers
7. Swapped the GMA340 for another one, noise still occurs
8. Noise is not the ELT
9. Powered off each radio by the radio power switch and noise still occurs
10. When pilot isolation is presses, noise stops in pilot position only, noise still occurs at all other stations
11. Noise stops when speaking in intercom or when radios are receiving audio but returns when all becomes quiet again
12. Unplugged all headsets and tested each station noise still occurs
Both me and the avionics shop are stumped so I turn to the wisdom of the board....
Thabks for you ideas.