Crostfire2,

The first thing to try is check your grounds.

(Are your RCA's wired down the same side as your power wires?) If so run them down other side.

If your Polk passive crossovers are installed near a computer or power wire you maybe be picking up radiated noise.

You may also have a bad rca.

Try not to get frustrated. If you think it is the fan pull the fan fuse and take a quick listen to the system.