s60 t5 radio quirk

i cant really figure this one out. there are sometimes when i turn the car on (either on position or running), and you can hear a beat (about 2 times per second) through the speakers. i have noticed that if i turn the stereo off and the car off, then back on, but leave the stereo off, the beat stops, but once the stereo is turned on, the beat starts again, and wont go off until the key is actually turned off. it does not do it consistantly, and i have tried pushing every button in the car thinking something else might be affecting it, but i have no idea.

matt

how close is your cell phone. try turning off the cell phone