Would anyone have an idea about the cause of this?

Problem:
Intermittent noise (sounds just like a squelch volume test) on both
communication radios. Lasts from 15 seconds to two minutes. Will
occur at any RPM including idle. Changing frequencies eliminates the
noise but the noise can and most likely will come back shortly on that
same frequency.

I have a back up alternator on this aircraft and when I have the main
alternator turned off to check the operation of the backup alternator,
the noise never seems to occur. Also, when the noise manifests, if I
turn off the alternator the noise always goes away. I have done this
several times on the ground and it has been 100%. The voltage
regulator has just been replaced and the noise is still present. The
alternator has been rebuilt three times in the last 30 hours of
operation. The alternator shop is tired of hearing from me. This
squelch break noise is new however in the last 15 hours of
operation.

The alternator has been having so much trouble because we have been
trying to locate an alternator field circuit breaker popping issue.
Typical scenario is the aircraft must fly for at least 40 minutes
before the breaker will pop. Usually, it pops in conjunction with a
small static crack in the headset and a voltage excursion of one to
two volts. This most often happens in level flight. This same
voltage excursion has been noted at least once when the main
alternator is turned off and running on the backup alternator.

All connections in the alternator/breaker/battery/ground circuit have
been cleaned, tightened and cable ends replaced when suspect. All to
no avail. This is a 24 volt single battery system. Running the 35
amp hydraulic pump to actuate flaps or gear will on occasion trigger a
field alternator breaker pop but many times it won't either.

Thanks in advance for any light you might be able to shine on this
problem.