Look for a bad ground ..  I had one found that the grounding system had a corrosive connection. 

Recently, my repeater at 42690 began developing an audio ' hum ' . An
Asterisk restart does not stop it. A reboot is required.
It is NOT power supply related. I tried substitute supplies for the Pi and
Repeater. The message log shows not related errors.

It is software related somehow.

[ short audio file attached ]

Len
WB2HKK

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