Truck Forums - strange noise

In 2005, I purchased my '03 F-150 FX4 from Woodhouse in Blair, NE. It was used with only 27,000 miles. I now have in the neighborhood of 66,000 miles on it and although it runs and operates just fine, I've been hearing a strange noise.

The noise seems to be coming from the vicinity of the frame rail under the area of somewhere between the gas & brake pedals to under my seat. When I'm coming into town I let off the gas and coast from 60 mph down to 45 mph. I hear the noise then. I also hear the noise when sitting still and idling. Both while in drive with the brake on and while in park with no brake on. I took it to the dealer and they can't pinpoint the problem. I crawled under it the other day and located what looks like a fuel filter along the frame rail. Just in a little bit more, I don't know if this is the transfer case or the transmission but there is a couple of round module looking devices with wires coming out of them attached to this. These are in the correct area as far as the origin of the noise. It's the same type of sound that my trusty Pinto used to make when the battery was about dead. It's a selonoid type sound.

The sound I hear is similiar to if someone was tapping on the transmission with a ball peen hammer. Sometimes it is once and other times it is 3 or 4 taps in quick succession.

I sure hope someone here has an idea as to what the problem could be, the mechanics at the dealership didn't have a clue.