Hmmm....

My '06 Cayman S (~72.5k miles) has a rough running spot between 1800 and 2200 rpm. While there's no perceptible power loss, the noise is a bit disconcerting. The best way I can describe it is like when someone cracks open a window in a sedan going down freeway speed. There is a low pitched reverberation that is not there at idle or above 2500 rpm. It doesn't matter whether the car is under load or not.

Recent related maintenance:
New battery: 72k mi
Oil Service: 71k mi

I replaced the AOS back in 2018 at ~58k miles. Spark plugs were last replaced ~39K in 2017.

I haven't taken the engine cover off yet to see if the airbox is wonky or whether the AOS is having vacuum issues.

When your AOS was changed was the throttle body also cleaned?

Yes, I cleaned the throttle body when I changed the AOS. It's at the shop now.

You say there is a low pitched groany vibration but no discernible loss of power? Is there an obvious change in RPM? Any other engine related symptom? If the engine runs normally - no loss of power, stable RPM, and consistent delivery under load vs off - that sounds more like a mechanical problem more distant than the engine.

I think I might start with the tires. Make sure they are in balance and not cupped or something. Then think about wheel bearings. I might then think tranny or clutch.
All the above said - diagnosing a sound over the internet is darn near impossible. Even in person, it is more art than science. Experience mean quite a lot.

Sounds and symptoms appear regardless of being in motion or a standstill. Doesn't matter if the car is in gear or not. Symptoms don't seem to change whether the engine is cold (fresh start) or already warmed up. Possibly actually worse when warmed up.

There is no RPM change, I can hold a constant RPM in that range without adjusting the gas pedal. There is some valve train noise that is more similar than my 914 than the Cayman, but I could just be getting hypersensitive.

Hasn't thrown an CEL and an ODBII reader doesn't show any misfires or any other codes.

Yeah, didn't have much hope is being able to diagnose a noise issue online, but was a shot to see if anyone else had experienced this given how weird it is.

Regardless, here are things that are going to get checked out:
- coil packs, they're original, so probably EOL of those and should get the updated ones on anyway
- engine/trans mounts
- something wonky with the Varioram actuator (although that should throw a CEL)
- something loose along the intake or exhaust path

Remove the firewall behind the seats and remove the poly-ribbed belt.
I would check: the Water pump. The bearing can make that sound you describe and your mileage is close to the low limit for the pump.
Also check the free-rolling action of the Idler and Tensioner rollers .
Happy Porsche'ing,
Pedro

Pedro Bonilla
1998 Boxster 986 - 311,000+ miles: [www.PedrosGarage.com]
PCA National Club Racing Scrutineer - PCA National HPDE Instructor - PCA Technical Committee (Boxster/Cayman)

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Racecar spelled backwards is Racecar

"Racing is life. Anything before or after is just waiting" ... Steve McQueen as Michael Delaney in "LeMans"

"If you wait, all that happens is that you get older"... Mario Andretti

"Being second is to be the first of the ones who lose" ... Ayrton Senna

And the answer is engine and transmission mounts.

Front mount had obvious cracking in the rubber. The transmission mounts had nothing visibly wrong, but after only replacing the front, there was no real change in the issue. Replacing the rears, things are now significantly improved. There's still some slight vibration/droning, although, almost unnoticeable and that's probably a result of the dual-mass flywheel getting out of balance or worn. Will take care of that in 10-15k miles and do the clutch and shift cables.

Thanks, all.

It strikes me as unusual that the noise didn't change as between moving and not moving.

One would think the torque of propelling the wheels - or the reverse torque of deceleration would have a noticeable effect. Curious....

But I am glad you have a relatively benign resolution.

No go enjoy the thing.....

My guess is that my hearing was not sensitive enough to notice a difference. But the difference of before vs after is unmistakable.