is it a sqirril, is it mouse, is it a thief. no its a radio fault?

The radio is off , all is locked up when theres a scratchy bumping coming from my Ducati camper,after an initial doubt it turnsout to be coming from the radio speaker all 4 .
Switch the radio on and all works fine.Switch off and after a few minute it starts again,
Iv had the radio out and the noise continues un till the fuse is pulled. Had the radio apart no obvious caps blown, burns or marks on the boards.Nothing is effected when I move the wiring.
Obviously there must be some current short between 12v radio and the speakers when the radios off, The ISO connection has live 12v on several pins
Has any one had this and have a sugestion
The radio is a Beaupunk london with cd