Always on the same mic? And only on that mic?

If so, I'd call it a loose connection inside the mic (or on the connector).

Thanks for the reply.

Yeah its always on the same mic and only that mic. The head does look slightly skewiff compared to the non humming mic.

Is it best to send it to AKG too look at or could i fix it myself?
Thanks

One thing I'd check is the battery tabs, make sure they're clean and both making firm contact if there is a battery present. If you're using phantom power take the battery out. On rare occasions I have seen phantom power interact with the battery in a bad way when they don't make good contact.

They have a 3 year warranty - any chance they're covered?

The C1000 is obviously made to screw apart to change batteries and filters, make sure something didn't get pulled or crimped inside. The good news is, you've got a perfectly good unit to compare it to.

If you don't see anything out of the ordinary, I'd take it to the dealer - unless you've fixed mics before. As a rule, mics are deliberately difficult to take apart and usually use rather small wires inside that are none too long - not for the average DIY type. Odds are you may do more damage just trying to get it apart.