So I went driving to see if I could replicate the issue. Valve train sounds fabulous and smooth. After it warms up, car sounds great.

Driving on the way back after some twisties I started hearing "That noise" if you bring the car up to 3k and just let off you can hear the weird rattling noise. I could actually FEEL it slightly through the seat. I thought "What in the hell?"

Soon as i got out to replicate it... the noise was gone.

I'd hear it again... rev it, hear it... then open up the engine lid and again it would not replicate itself. I then put my hand on the engine praying to the Porsche gods that it was not an engine issue. Again, no sound.

Finally... I had my wife step on it while I was outside. I could hear it but could not pinpoint it. And it was quite audible and i could FEEL this weird noise throughout the car.

Finally, popped the engine lid... had her do it again, no noise. Put my hand on the engine lid after we drove it around for a few... AHA there it is! Open it up... gone...

So... I must have it rattling on something on sudden rpm shifts.

My 944 does something similar with a bracket that rattles at about 3kpm and it drives me bonkers but I said fuck it I don't feel like messing with it.

Aside from that... the car sounds fine. I'm going to go drive it.

Thank you to everyone who told me to just stop worrying and go drive it. It did end up being "Just fine".

It may just be something simple like a loose bolt/screw or spring, latch, etc. on the engine lid (since it comes & goes with that open/closed), or loose bolts/connectors on the tins, HE's, muffler, etc. that "sings" when you hit it's resonant frequency.

Maybe do an overall check-over for loose bits next time you work on 'er, unless you can track it down before then.

Same with that 944 bracket, which may just need a thin rubber/plastic washer to isolate the vibes a bit!

Start with the simple stuff & work up from there, rather than worst case major items requiring the intervention of the Porsche engine gods!

My air box inlet tube was rubbing on my rear lid torsion bar making a loud vibration noise. I went through the same thing. I could only hear it driving. The noise wasn't there when I opened the lid, because the tension was off the bar(s) eliminating the contact.