Cool Beans: Audio loop. Ditto what Ray wrote.

CND: Ground loop/mains line noise from two, or more, poorly isolated AC/DC power supply/converters. Switching to the turntable's phono (RCA jack) output powers everything from the external preamp only. Must be a dedicated low level RIAA type turntable input though. Not always standard as vinyl began to fall out of fashion.

Laptops and PCs sometime don't play nice either. Different +/-DC rail voltages. Floating the laptop on battery only will usually work around that.