It is possible that the incomming power from the electric company is noisy, or another device on the same circuit is adding EMI noise to the line. Even DVD players have been known to do this.

Try removing anything plugged into the same circuit except the sub and receiver and see if the noise goes away.

Also, you could try plugging everything into a line-conditioner. There are some affordable units from PureAV (Belkin - http://www.pureav.com/power/) available at most B&M stores, as well as upper end units from companies like APC (http://www.apc.com/products/apcav/) designed with home theater in mind.