Peavey CMC-1 hums like an airplane

[tilze]

I picked up an 80's Peavey CMC-1 Companded Chorus pedal not that long ago. When I got it, it wasn't grounded properly to the casing. That was an easy fix with a couple washers, but now when I turn it on, it hums like a prop engine. Any suggestions for the root of the problem?