The same thing happened to my F113. Lightning struck very near our home and the sub started squealing immediately. It was plugged directly into the wall as well. I had to send it back to FL for repair. They suggested a SurgeX suppressor/conditioner.
JL Audio F113 --- Loud Buzz --- ??? --- Bizzarre
I have two F113's in my 2 channel system. I keep them in the Auto Mode so they turn on when they sense a signal. All of a sudden one starts blurting out this high pitched rumble that ends up stuttering loudly. Turn the power switch to off and obviously it stops. Turn it back on and it emmits the same sound and it doesn't stop. I have tried other power recepticles, unhooking IC's that run to the pre amp. No success ......... what the **** happened ???? Bizzarre ????