That sounds like fuel tank cradle above... Does it vary based on tank level ? it was a cheap fix ! couple hundred $.

I had a (very loud and annoying) rattle from almost Day 1 that I thought was from the dashboard on my 2017 991.2 GTS... dealer (Reading, UK) fixed it and said it was something to do with lubrication on the front suspension. I am no expert so I can't vouch for the accuracy of that. One way or another, the rattle disappeared.

Had similar rattle that I could not tell if was coming from the passenger side outer dash or the front of the passenger door. Was nearly constant but got much worse with deep bass notes of the stereo. Took it to the dealer a couple times and they did the TSB with the felt and all... no better. Later when I was installing Nick's (Musicar) group purchase DynAudio tweeter setup (HIGHLY recommended BTW!) I found a broken tab on the passenger side AC vent. I JB Welded it temporarily until I could get it to the dealer and that fixed it. They did go ahead and replace the AC vent...

It does vary. I filled up the tank and it showed up again. I'm going to monitor on this tank and see what happens. Thank you for giving me a place to start!!

My rattles are the same with fuel tank full or nearly empty. Since the dealer attempted the fix, the rattles occur during moderate to heavy braking or when driving on rough roads.

My 2013 Cayenne had a similar rattle/vibration emanating from the left side of the dashboard. I initially thought this was coming from the left driver side vent. However, it was ultimately determined that the Z wrench in the fuse box was loose and was vibrating against the cover. Once the wrench was secured, this very annoying vibration was gone.

I have a very similar rattle in my 991.1

Did you end up solving the issue?