Assuming CR-620 is an amp or receiver. Is the noise on wireless or everything ?
If it's on everything and doesn't increase with volume I'd look for a knob or lever which switches between two pairs of speakers. Try flicking it on and off and see if it causes the problem. Ditto tape monitoring switch.
If the noise increases with volume, check input switching and damaged input connectors and connector cables.
Just to be sure check the speaker wires and connectors at speakers and receiver ends.