My SBT running 7.8.0 r16754 has started a low level hum / buzz on its audio outputs, almost like a ground loop effect.

Nothing changed in the in overall system.

The buzz/hum noise only kicks in once the unit is past the initial Logitech boot logo.

The sound is present on digital coax, spdif, and RCA. It more noticeable/louder on the digital outputs.

From time to time there is the odd crackle and pop.

The SBT uses ethernet, WiFi is not connected.
The SBT is on the original OEM PSU.

I don't think it's a group loop issue with the amp, as the problem happens over optical, when no coax/RCAs are connected.

Any ideas? I've done a search but can't find anything similar. Could this be just a PSU issue or more drastic? It's odd it's on digital and analog outs.