I don't know if this is what you have, but I had a similar noise in the '55. Drove me nuts trying to find it. After I checked the cab mounts, bed mounts, etc., I found that it was the lower seat cushion. The screws had come loose on the drivers side. Evry time i hit a bump, the seat would knock, even with my big butt in it. Might be some thing to check on yours.
