After replacing motor mounts and trans mounts along with the exhaust-mani gasket I narrowed my search for this annoying sound.

The sound frequency becomes higher and "buzzier" as the motor RPMs increase *** the sound is coming in from the air-intake.***

I checked to make sure the air cleaner assy wasn't rattling, okay.

If the motor is at a low-idle, it thumps, but as the RPMs increase the frequency does too, to the point where it sounds like an outboard motor buzzing. It's not a mechanical sound, but purely air-flow related.

If I take the air cleaner out etc and interrupt of the flow of air with my hand, the noise diminishes/changes. But the sound is coming from inside the air-intake- or air going into the motor, not the air-intake ass'y itself.

Also, I noticed the PO wrapped the plastic air-intake tube with a foam sound-deadening material- which does mask it to a point.

The motor makes no strange mechanical noises, it runs fairly smooth, it rocks a bit however. Shifts out great, no excess oil use, excess smoke etc for a non-rebuilt 180k miles motor. I get 28 on the hwy...

I wonder if I have bad compression on a cylinder and the noise is air-compression related??? That's my first thought if it makes any sense.

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'07 Yukon 2500
'13 Subaru Outback 3.6R
'13 Orbea Carpe 9-speed

Currently Benzless
Formerly: 300TD, S600, E55, 560SEL

---= The forest breathes, listen.
-Native American elder

I dont know if it applies, but have you checked your turbo for excessive play? or the air cleaner mounts (again I dont know if this applies)

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hum.....
1987 300TD 311,000M Stolen. Presumed destroyed

It's NA.

A '79 300td wagon, probably should have mentioned that.

But it isn't air cleaner mounts. I can take the air cleaner off and here the noise coming from intake area of the motor. The sound is made by air rushing in at a certain velocity. So, if I put my hand in the way, it'll quiet down.

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'07 Yukon 2500
'13 Subaru Outback 3.6R
'13 Orbea Carpe 9-speed

Currently Benzless
Formerly: 300TD, S600, E55, 560SEL

---= The forest breathes, listen.
-Native American elder

hole or debris in the intake line?

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hum.....
1987 300TD 311,000M Stolen. Presumed destroyed

Nope, it's coming from inside the motor. I can take all that stuff out of the equation... it's how the air is behaving in the motor (if that makes sense, lol)

__________________
'07 Yukon 2500
'13 Subaru Outback 3.6R
'13 Orbea Carpe 9-speed

Currently Benzless
Formerly: 300TD, S600, E55, 560SEL

---= The forest breathes, listen.
-Native American elder

When was the last time the valves were adjusted? It could be an intake valve hanging just a bit or possibly cracked. That would also explain your slight shaking at idle.

That makes sense. I think it's been 10k miles or so.

__________________
'07 Yukon 2500
'13 Subaru Outback 3.6R
'13 Orbea Carpe 9-speed

Currently Benzless
Formerly: 300TD, S600, E55, 560SEL

---= The forest breathes, listen.
-Native American elder

There may be a gasket that has failed. Bottom of the air cleaner?

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83 SD

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