High-pitched noise when car on or off

Beginning yesterday, we began to hear a high-pitched noise apparently coming from the dash in the vicinity of the radio when the car is both on and off. I opened the hood while the car was off and did not hear the noise in the engine compartment. I removed the fuses for the radio and the instrumentation panel lights, but no dice. Anyone ever seen this problem before ?

Never seen it, but I've heard it.

Ghosts!