Noise in headset

[Cruiser]

During the past 10 to 15 hours of flying I have noticed an increasingly frequent snapping, crackling and popping sound in the headset/intercom. 

Also seems the electric tach is starting to become erratic in display of RPM. Originally it was only at low RPM and cruise readings were steady. 

On the last trip the crackling and pop has gotten to the point of being distracting and the RPM gauge is now erratic at cruise RPM as well as low readings. I noticed the RPM reading jumps in relation to the noise.

 

On shutdown I turned off each unit until the noise stopped when I shut off the transponder. Turning the transponder to STBY is quiet, Turn it ON and the snapping and popping start about the same time the interrogation light starts blinking. Turning to ALT no change. Turning the transponder to STBY and the noise stops.

 

So I shut down the engine and turned everything off except the MASTER and the transponder and I get the above results.

 

Anyone else experienced this problem? Does it sound familiar? 

 

Transponder is a B/K KT76A.

[N201MKTurbo]

The KT76 has a vacuum tube, thus a high voltage supply. It sounds like it may be breaking down and arcing.

 

Time to visit the avionics shop

[Txbyker]

I had crackling in the headset. Turn off strobe and it subsided. Press lean find button and its back. Turned out to be bad capacitors in mag. Replaced the cover and wires and all of the static gremlins disappeared. But with engine off that may not help.