I started my '83 F150 4.9L six and after a few minutes, it began to clatter.
It started doing it on the freeway.
It is audible at idle and while accelerating.
It seems to run as always but I'm afraid something might break.
I thought I heard a vacuum leak, before the clattering started.

Wonder if it might be timing chain?

 

If you have a 3 foot piece of pipe use it as a "ear extender" and stick it to the lifter cover plate. I think you will find them to be the source. Its possible you have a stuck lifter.

 

This turned out to be an A/C belt that chunked off and caused the noise.