azurita, is this a new amp (under warranty)? If yes, take it back to the dealer for an exchange.
If you bougth it used then I would check two things. First the tubes, an easy test is to swap the tubes if the "noise" moves to the other side it is the tube. The second thing I would check is the quad phono socket, the leads in it break sometimes and it will need replacement. To do a good inspection you will have to unsolder it.
If neither send me a PM. Good luck.