high pitched noise when electric switched on...

pretty much as the title says really. we took our caravan out for the first time last weekend and love  'jeremy' to bits. Hes an Abi dalesman 520.....the only thing is when we plug into the electric hook up and switch on the 12 volt electric switch we get this really high pitched noise, its a big pain in the bum as we obvoulsy switch it on for the  water pump, telly aerial , lights etc.....anh ideas guys its driving me maaaaad! xxx