Crate guitar amp buzzing

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Hello gents, im having a perplexing problem with this amp, after transporting in car the amp is humming/buzzing when a guitar lead is inserted, Ive checked the guitar's ground connections and all seems ok, dont think its the guitar because it hums with just the lead inserted into the amp, but not with no lead.

Ive tried multiple leads, and when plugged into guitar touching metallic parts reduces the hum slightly, though it still very noticeable.

When a microphone is plugged into amp, theres no hum and it amplifies perfectly...

This seems like a grounding issue, but where exactly? Any why not with microphone?

Cheers
Colin
 
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