Seems that just about every water-cooled 911 I've ever heard is on the clattery/mechanical sounding side at idle (not counting exhaust sounds). I've always written it off to the cylinder heads being so low and reflecting sound right off the pavement vs being tucked up at the top of an engine bay near the hood's insulation as in most cars. Now a scored cylinder is a different story altogether...

 

When I bought my car I thought it sounded great and always wondered why people would ever want to change out the exhaust.

Then one day I looked closely at the window sticker and noticed it had PSE installed. Aha! That explains it....

 

I'm getting a new motor due to cylinder scoring on my 2015 991 S2.