exhaust heat shield?

maybe the valve noise. Had to tell without hearing the noise.

Take it to a mechanic to check it. 

In addition to the excellent answers above, make a video of the noise and post the link here. From your initial description, I’m with @mountainmanjoe on this one. 

Either that or some loose bolt somewhere. 

Go underneath and start banging on the chassis exhaust etc to mimic the engine vibration, the rattle should be audible. 

I'm surprised you haven't done something like this already.

Start the engine, put it on a lift, grab a funnel and put the small end near your ear and walk around under the car, listening and isolating where the sound is coming from.

I work for a used dealer, our mechanics do that all the time, in 60 seconds or less they know just the noise is coming from.

But you can "try this, try that" if you really want to.

If nothing is loose on the outside of the engine, I would say it is probably a broken timing chain guide. If one breaks, the chain will be loose and can rattle. But what makes me not 100% on that is because you said the noise has been there for years. A loose chain should have gotten worse by now.

Wow! Thanks for all the replies so quickly...I was thinking it would take days to trickle back to me....does the timing chain loosen as RPM's fall? and tighten when throttled? anyways, the car (like I said) runs silky as butter....1 owner...old man....then me. heheh-

I will have to get it checked I guess....been blowing it off because of funds....but it is equally starting to weigh in on my sanity

all I know is if I rev it...she goes "vroom".....and as the rpm's fall back to idle...this rattle, clanking...slapping sound kicks in.

Shes my baby....gotta get her fixed...! and thanks for all and any further help....I realize there's only so much you can diagnose from looking at words on a screen, but so far you are all so kind and helpful

good lordy poo....thats a pretty hefty fix if thats what im facing. tax return? STIMULUS?!?! hahah 🙄