Something buzzing at idle

[Bitsyncmaster]

I've had this problem as long as I've owned my car. When the engine is a certain temp I have something buzzing (sounds like metal vibration). I can not determine where it comes from. Sounds like the rear (bumper side) of the engine. I've used a screw driver to place on things like the muffler heat shield, alternator, water pump etc. with no change in the buzzing. Then suddenly it will stop.

Anyone had this problem and found the cause?

[Ozzie]

Loose, or missing screws, on air deflector plate?

[Bitsyncmaster]

Loose, or missing screws, on air deflector plate?

Air deflector plate? Do you mean the heat stove?

[David T]

Check around the muffler and especially the right, rear muffler bracket by the alternator for cracks. I hope you are not referring to the frequency valve! That is supposed to buzz!
David Teitelbaum

[DMCMW Dave]

Crossover heat shield, or heat stove on RH manifold. Make sure it isn't the flexible heat duck dragging on the water pipe below the header bottle or you will find the problem eventually. . . .

[Ozzie]

Air deflector plate? Do you mean the heat stove?

These guys:
https://www.delorean.com/store/images/PRODUCT/medium/106733.jpg

They're thin enough and near major resonance input, to vibrate as you describe.

EDIT: ...but this would be more frequency (resonances), not temperature, driven.

[Bitsyncmaster]

Thanks for the suggestions. I wish it would do it every time I'm warming up the engine than it would be easier to find. Since I've been warming up the engine at idle many times most every day testing my ECU it has shown up a few times.

Guess I will put the back end up on ramps and hope it does it then. Maybe I should just remove that heat stove (it sounds like a thin metal vibrating) and the thermostatic air diverter valve.

My plates over the trailing arms have all the nuts on them but I could check the buzzing when it does it again up on ramps.