The only way to locate unwanted signals is to kill everything one at a time in the house. You would be surprised as to all the devices that generate crap. Don't forget to pull the plug on the TV, the cable box, sat. receiver, any Ethernet devices and all computers.
If you have more than one scanner, kill the ones your not trying to solve the problem on. Some clock radios will generate signals like you would not believe.
Like I said, you basically need to pull the main power breaker on the house and start by turning on one breaker at a time till the problem comes back. Bear in mind that it might take several minutes for the noise to show back up.
Run the scanner on batteries while you try the entire house killing the power. One breaker off at a time is the best move. Like I said, it may take several minutes to have signals come back after the power is restored.
Good luck hunting for it. Don't forget that the problem might not even be in your house. It could be coming from several house down the street. Once had a problem on a public safety frequency that was over a half mile from the dispatch station.