Hissing from throttle body?

[far2wrecked]

After having three breather pipes replaced (cost £120 parts only, ouch!) on my 2.0 petrol passat. I am now getting a hissing noise from behind the intake manifold on a fast kick of the accelerator, which I've located coming from the throttle body. All the pipes in this area are leak free.

I'm pretty sure there was no hissing before, is this normal as the old breather pipes were badly split, the intake manifold also had to be removed (its plastic not aluminium -rubbish) to fit one of the breather pipes. Any advice on this problem would be greatly appreciated.

[far2wrecked]

The garage that fitted the breather pipework checked for leaks visually and with spraying brake cleaner, no change to the engine idle, so he said there isn't any leaks. But today I noticed that when slowing down, changing into neutral gear the engine idle sat at 1100 rpm for 10-15 seconds before going to normal idle position. Since the pipework was replaced the mpg values on the dash have also became erratic. Again I may be getting paranoid as I had a nightmare with my previous car.