Also, if your running Cable TV, try the set with an antenna (if close to the city). If the noise doesn't happen when on the antenna, there could be a poor connection in the cable, or the AC electrical ground and neutral bonding.
If there is actually a bad AC power ground/neutral connection, your place can be grounding through the cable TV wire. This can be dangerous too.
A boiler, furnace, can cause interference.
If a light switch is turned on, and doing nothing, why can't you turn it off?
Check the real estate laws. A lot of states have disclosure laws. If there was a known problem, the previous owner may be responsible.
Maybe an electrician will post with ideas.