[Darryl Ramm]

Very first post described the clicking sound associated with engaging the HPF.. it's likely the relay on the audio input board.

Trivial tests for somebody handy with electronics. Confirm that channel relay is clicking (there are a ton of relays in these boxes). Check power supply voltages. Try to swap the input/output cables on that input channel with another channel. The channels have multiple electrolytic caps, which well don't last forever so that's an early electronic thing to always exclude, and if so then check other caps you might end up wanting to recap the whole box... or maybe just buy a new interface. All pretty obvious stuff to anybody in electronics. Or just don't use that channel...