https://www.allaboutcircuits.com/textbook/direct-current/chpt-14/electromagnetic-induction/
The AC power changes the magnetic field and may be being picked up in your amplified leads .
Or the DC voltage in your amps power supply is not filtering the power after it rectifies the AC power .A really big capacitor across the dc voltage output from the rectifier should filter any AC out.
https://www.allaboutcircuits.com/textbook/experiments/chpt-5/rectifier-filter-circuit/
Pedals operate at a different level then rack units that go in the effects loop.
It needs to go in front of the amp input. Guitar to pedal to amp input.
Normally on older Peavey amps the grill is on a frame attached to the face held on with velcro. I suspect you have something vibrating against the speaker if it's only one frequency. Once the grill is off it may be apparent.
Also on old Scorpions and Black Widows Peavy has a piece of foam in the hole in the magnet to keep junk from getting in the speaker. Over time it crumbles and opens that hole, but even worse can get it the gap and freeze the voice coil. I always suggest cleaning it out while the speaker is out of the cab.