Not true. If the RCAs are shielded (the main conductor is run inside the outer connector) there wont be any noise if as long as the outer conductor is grounded.Your power cable being too close to the RCAs definantly causes noise. The reason is simple. the Power wire sets off a frequency like all other electronics, if the RCAs are too close they will pick up the waves and boom you get noise.
Back to the orignal problem, sounds like you might have disrupted the ground when you installed the cap.