I wanted to provide an update on this thread. I have narrowed down the issue I described above to likely be caused by timing gear separation from the intermediate shaft. I can discuss how I narrowed it down to this if anyone is interested, but I have eliminated the possibility of failed lifters, spun cam sleeves, worn oil control rings and housings, defective cam phasers, defective solenoids, worn chain guides, stretched chains, and failed chain tensioners.
Timing gear separations are apparently a rare occurrence, but it happens when the bolts holding the timing gear to the intermediate shaft come loose. This issue is possible on most Mezger engines, but so far have only seen this discussed on 996TTs. See these links where other people have documented this same issue.
Here is a photo I found of this same issue taken from the link below:
http://www.911uk.com/files/p4121363_874.jpg
911uk.com - Porsche Forum : View topic - 996 turbo IMS failure
https://www.pistonheads.com/gassing/topic.asp?t=1121214
Similar to what was described in the above links, the rattling is most prominent when the engine is decelerating from around 3k on down, and when the intake cam high lift is activated.
Unfortunately, the solution for this requires an engine teardown and rebuild to replace the intermediate shaft and gear use good Loctite on the retaining bolts. A better and more expensive alternative is to use the intermediate gear set from the GT3, which is a one-piece part.
In my case, I will eventually have to dismantle my motor to confirm my suspicions. For now, the car will not be driven.
Has anyone else encountered this issue on their 996TT/997TT motors or any air-cooled motors, and could you please contribute your experience here?
Thank you,
Jeff