Working on my Univibe pedal.

[Simon Barden]

It's not going to be a ground loop, but it may just be some bad grounding or lack of full shielding from the case.

It may also be that it doesn't like the hotter signal level in the FX loop. It's a nominal -10dBV, so consumer line level. Guitar signal levels are around -20dBu (note the different units), but vary depending on how hot the pickup is. But it also depends on how much headroom the pedal has.

Worth spraying some contact cleaner into the FX return and send sockets. As it's a series FX loop, they are switched contacts and if not regularly used, they can corrode over time which can affect the signal and make things like hum worse.