[lurkingknight]

shimming is off. Why you don't hear it when you have it directly plugged in is because the fuse and extra connector add resistance to the electric circuit.

Sometimes when things run slower, you can hear the issue better. It's like a wheel out of balance on your car.. you can only feel it at certain speeds go faster or slower by a bit and the resonance goes away.

Fix the shimming.

Then fix the wiring.