I installed above mentioned exhaust, I like that is loud, but at some points it sounds as if it has a leak, farting sounds. Everything is bolt on properly, so I wonder if this is normal for Porsches? I have 82mm TB and plenum and nothing else.

 

Probably is leaking a bit, most of the exhausts/downpipes I've installed leaked for a bit till the carbon built back up around the joints. If it doesn't change in a week or so (daily driven) then I would recheck it. Normal.