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I have an Allums modded sd-1,rocktron metal planet distortion and a dod flashback fuzz.
Now when i'm playing guitar through the overdrive setting on my marshall amp everything soundwise is ok but sometimes when i switch on either the rockton or the sd-1 i get a background whistling type of a sound almost like a type of radio interference.It doesn't happen when the dod is on just the other two.
Anyone have any suggestions what could be causing it?

What guitar are you playing?

Mustang at the moment but it's done it previously with my bronco.

It could just be noise due to using single coil pickups, you could try shielding if it's a real problem. My Strat used to do a similar thing with my EHX 22 Calibre, but I don't have the same issue with my Jazzmaster which I had shielded.