BTCC A4

I got a new MAF and then this happened. ( I was running with a broken Maf for a long time).
When I installed the new MAF the car would seem to run fine until releasing the throttle, as the engine will stall at stop lights or have rough low or fluctuating idle speeds and exaust would smell like crap.

I suspect I have a leak in the throttle bypass system. The reason I say "throttle bypass system" is because as far as I can see, there doesn't seem to be a throttle bypass valve on my car, I think the idle is controlled by that disk shaped vacuum/spring device connected to the throttle rod.
1) Is there a way to verify my theory?
Heres what I think: At idle if I try to close the throttle further I would be able to detect the leak because naturally the engine will still be trying to pull air into the manifold but the throttle being not as wide the only way for air to enter the cylinders would be the vacuum leak, which would presumably cause a hissing sound or cause a lean mixture.
2) Does this sound correct?

And with the engine off, I tried pushing the
disk shaped object (causing increase in pressure) and I heard a hissing sound somewhere not on the disk. I tried to follow the sound but the rubber hose disappears under something. Is this normal?
Does this sound like a vacuum leak? Cause I would think the other end of that hose is connected to somewhere on the manifold where negative pressure is generated.

By the way this is on my Silver A4. 98 1.8T

Thanks,

needforspd

but I have no ideas as im still new to the Audi world of mechanics ...

Had a guy say maybe its my TurboXS ByPassValve not adjusted correclt? or Teddy said might be the boost controller connection in the engine.

Audi2ptzero

Is your new maf using the stock maf housing or in a larger one?

BTCC A4

Its the stock one