Help.. wired noise, misfiring

[Harvey]

Hi there, first time posting here.

I have a 2011 Audi A5 2.0 TFSI with 85k

A bit of backstory.. My cars timing belt went while driving and was replaced in January. This was caused by the timing belt tensener becoming slack. After that my car has been struggling to keep constant revs when cold and ended up misfiring on a long drive. I managed to limp it to my garage where they changed the O2 sensor. The car seemed fine untill about a month ago when a wired metallic rattle started when letting off the gas. Sounds a bit like a clockwork spring being released. At first I could only hear it after turning the engine off but has now got louder. Sounds like it is coming from the left side of the engine. My car has started misfiring again and is currently undrivable as one of the cylinders isn't working. It also has a fault code saying the engine is running to rich at idle. This has gone after both repairs but returns after a few weeks. I had a fault code last year saying the car was running to lean but went after a week so haven't been to worried about that. I hardly drive my car. Probably around 1000 miles this year. 

Could anyone provide a bit of help. Just want to sell it now but need to get it working first