1990 460 5spd. Can't hear anything from the engine bay. Just underneath truck. Kinda sounds like transmission? Only does it under light load like cruising on the highway.
solved: spark knock

 

sounds like the trans.

 

Sounds exhaust related to me. Does it have a catalytic converter? My converter makes a similar noise occasionally.

 

My cat does too. Usually can't hear it once it's moving but you can at idle for me.

 

Go underneath your truck with a dead-blow hammer or a rubber mallet and start tapping on parts til you find the one rattling. It doesn't sound like transmission. Sounds like exhaust or sheet metal like a skid plate or heat shield.

 

it was spark knock.