The 501 really isn't that much of a radio )-: I'd be very surprised if the noise was coming through your power line , in most cases , it's just not that easy )-: more so then not it comes from the antenna , you can do a test with your car running and your radio on , unscrew the coax from your antenna to the radio , noise goes away ? then you know where it's coming from , you can do a lot of things as Mole dude suggested , but i really don't think that 501 has very good rejection what so ever ......if it were me , I'd just kick it up a notch and get a slightly better radio with better noise blankers and anl's in them such as a cobra 25 or 29 , they really do knock out most of the serious engine noise that goes through the antennas and it is a lot easier then trying to sniff out the problems from the sourses. Good luck to you and welcome to the forum.