Originally Posted by UrS4boy
What ignition system are you running? Stock or aftermarket? What spark plugs? (Some plugs cause radio interference).
If it works fine with the engine off, it's not the antenna, it's engine electrical interference (electrical "noise") aka RFI (radio frequency interference).
Have a read on this:
Stock ignition & plugs - I was the lucky guy with the butchered wire harness back in September.
The interference started abruptly while driving in the rain late last week. It definitely only affects radio reception and not sound from the head unit generally, and stations with closer/higher power antennas seem to come in as before.