2000 CLK320 Engine and A/C query

I have a 2000 CLK 320, with a problem with the engine:


The engine sometimes twitches in Park, Drive or Neutral (revs are around the 500 mark). I really want to sort that out but, dealer said needle in haystack, as no faults are showing on Star Diagnostics. I changed the Air Filter, no difference. Why is it shaking slightly, there are no problem if the car is creeping or moving.

also

I have a strange "gas seeping (hissing) noise out of the middle vents" occurring constantly when I have my A/C on, if I press EC then the noise goes away. So obviously it sound like A/C gas or something. My A/C seems to work fine, except this noise has now started.




Appreciate some tech help or fixes.

N