6505+ FX loop pops

[u/Gardunio702]

I have 4 amps running through an Ampete 442 which has the option to turn on and off the fx loop feature. I’m having an issue with my 6505+ head where when I engage the fx loop or switch to that amp with the loop on I get a nasty pop. I’ve tried moving other amps into that same position without any issue. So far I’ve only tried cleaning the send and return jacks with deoxit d5. I’m going to try changing out the preamp tube next (V5?) and if that doesn’t work I’ll let it sit for a while before I crack it open to check the board. Has anyone else had issues with the loop popping? If so please let me know what I should look for. Thanks