[Tim]

If your AM radio is able to operate on battery power, I would suggest you first turn off the main breaker to your home and see if the interference is coming from your home or from outside your home.

It is also possible that the interference is coming from an AC power line from outside of your home. Dirty or cracked insulators, faulty lightning arrestors or loose hardware on the power lines can cause interference. Power companies have specialized equipment such as ultrasonic dishes and hot stick sniffers for locating sources of AC power line interference.

Some other things to check:

Door bell transformers
Electric blankets or heating pads
Recessed ceiling light fixtures
Furnace control circuits
Light dimmers
Screw-in photocells
CFL bulbs
LED floodlights
Switching mode power supplies as used on laptops and other electronic equipment
LED under cabinet lights