Mystery metallic rattle

[MeToo]

So my '76 Fleetwood has made this metallic rattle under the hood ever since I had it (nearly 10 years and 20000 miles) I don't think its knocking or pinging etc. as the engine should have crapped itself by now. Timing has been checked and is all good and it has run on 91, 95 and 98 octane gas with no difference to the sound and its not temperature sensitive either. Plugs are also correctly gapped.

It only does it on light to moderate acceleration-if you accelerate hard it doesn't do it all. It won't rattle if parked and you press the gas pedal. It will sometimes do it on cruise control if you go up a rise and the car has to accelerate a bit.

Could it be the cruise control chain having too much slack? I notice when I engage cruise control there is an initial feeling of slowing down then a surge before it settles on cruise.

Just wondering what else it could be?
Thanks

[D.Smith]

Heat riser at the exhaust manifold?

[Chas]

My 1976 Coupe does the same thing!.........whenever it occurs, I just turn my hearing aid off.

[CadillacRob]

All the luck in the world to you in finding that.  Rattles can be hard to find.  The center emblem on my 50 Cadillac hubcap is loose and rattles over bumps. 

My 03 f150 made a bird chirp squeak when I hit bumps, especially in the rain.  For years it drove me nuts.  I had my wife drive around while I crawled around on the floor going crazy looking for it.  Then one day I was parked on a hill.  I shut my door and "chirp".  I could replicate the sound! Was not the door however.  I then proceeded to look like an idiot and slam my door closed over and over while listening.  Finally a friend helped me out and would you believe, it's the power steering fluid reservoir in the engine bay.  I still haven't "fixed" it but a few shots of wd40 buys me a month or two of chirp-free driving! Haha

Good luck to you!

[DeVille68]

Check the heat riser! Good suggestion.

My 68 DeVille developed a metallic rattle on deceleration. Turned out be the heat riser valve whose shaft was loose and rusted. On acceleration or enough rpm the exhaust gases kept the flap pushed open, but on deceleration the sudden stop of exhaust gases made the flap close and rattle with each exhaust pulse