Popping noise on FM radio only

I just installed a stereo into a 1995 Nissan Sentra, the harnesses had been cut off, and there are only 2 speaker, front door, that are hooked up behind the deck.  My problem is that there is a popping noise on FM only, AM and CD sound fine.  I have

• regrounded the deck,
• ran power wires directly from the ignition harness
• wired the deck constant
• hooked up a new antenna
• disconnect all speakers from the car and used a test speaker
• bench tested the deck

If the deck was in the car it makes the popping noise, but when the deck was bench tested it worked fine, even when I used the same temporary antenna that I used to check the deck while it was in the car.  The car's voltage reads 13.6v while it is running and 12.76v when it is not.  Any suggestions would be great because i have idea what to do. 

Thanks for all the feedback, i finally got the time to look at these posts.  My first feeling was the ground on the battery, but i wouln't have thought of the engine ground. 

To Stls_ron: it does it when the deck is sitting on the seat.

To Dyohn: it only makes noise when the car is running.

To Ravendarat: I tried replacing the antenna, and cable.

Again thanks to everyone for all the input.  Also sparkie, I told the customer to see about having his ignition system checked, but thanks for the heads up, i'll let him know if he ever comes back.