What is this pipe?

I had my airbox out earlier and noticed this pipe had some insulation tape around what looks like a joint. I thought the tape might have been where Skoda taped my carefully routed power cable back up. As I pulled the tape off, sort of moved the pipe a little bit, I think it may have made a hissing noise. I didn't pull the tape off as I realised it was a bit more substantial than insulation tape! :eek:

The joint I'm reffering too is further towards the back of the engine bay towards the bulkhead where the pipe is then clipped in place, this was the only pic I had though. What is the pipe and should a possible hissing noise be a concern!?