Long story short, just bought a 05 mk6 st with 90k on the clock.

Since giving it a service with all the filters/ fluids/ plugs and aux belt changed, it's developed and horrible metallic knock/ rattle or possibly pinking noise between 2000-3000 rpm coming from around the timing chain end. The noise isn't there when cold and gets worse the warmer the engine gets. Also ive had a lot of nasty smells coming from the cat when it gets really warm and that's when the noise is at its worst.

I've read the chains should be checked at 100k but being a every day car I don't have the time to take the side cover off fully to check tension but I have removed the cam cover and the chain is tight between the cam sprockets.

Another thing is the plugs when removed hot are very black and wet. The pistons are also wet which makes me think, is it overfueling and causing it the pink/ knock which could be causing the cat to smell?

Any help appreciated.
Thanks

Did you replace the aux belt tensioner at the same time?

Just the belt was replaced

pounds to peanuts its your tensioner. i had the same thing.

Ran the engine this morning without the aux belt on and the noise is still there, so that rules that one out.

Whilest the belt was off and engine running, holding the rpm up on the rattle and using a stethoscope it sounded like it was coming from behind the timing chain cover. Tried listening all over the engine and could only hear at that one place. However I thought I would drain the fresh oil to inspect and found metallic particals, which makes me think either the timing chain is making contact with the cover or I've got a engine bearing gone

You need to check the timing chain first of all, then again I don't think I've heard of one failing so far. Either way it doesn't sound good. Sorry!