Had a strange rapid clicking noise in the cab (90 gmc 1/2 ton Sierra 305) down by where the firewall meets the trans hump. It sped up with the engine so I started thinking clutch or bad bearing in the tranny (Getrag 5spd). As I was under the truck with a wood stick to my ear, contacting various components but couldn't pin it down. Then the stick slipped from what I had it on and came up againsed the two fuel lines (as they angle up towards the engine beside the trans. Bingo! The lines were in contact with the underside of the floor metal, just to the side of the trans hump.
It seems the rapid clicking of the injectors, or what ever pulses them makes a lot of noise in the steel fuel lines. I didn't realize throttle body injectors pulsed, thought they were a constant spray.
Anyway fuel lines were isolated from the floorpan and all is quiet.