[chittychittybangbang]

Because you say it sounds like leaking vacuum, it probably is. Use a vacuum tester and a pressure tester to test vac and pressure on the lines, even the ones you didn't replace. It's possible the big black ball, the vacuum reservoir is leaking. It has lines going to the brake booster in addition to vacuum pump and other places.

The MAF can be tested, look through the FAQ article on testing the mk4 MAF. For fine testing you need a VCDS cable to measure what the MAF is reading. I wouldn't replace it unless I knew for sure that it was bad or else you're just throwing money at the problem.

 

[chittychittybangbang]

Remove one end and put vacuum on the other end. Maybe the reservoir is leaking or the main metal hose coming off it is loose. If there isn't vacuum the N75 solenoid and the VNT actuator can't work. This is just going off your hint that there's a sound like leaking vacuum after you shut the car off.

 

[chittychittybangbang]

Yes, that is the reservoir. Not sure what thing you're talking about on the valve cover, pics would help.

Look through the FAQ article on boost leaks. It will give you some ideas and sounds. Hissing. The point of checking the vacuum lines is to make sure they aren't broken. One person reported that they had a chunk of carbon stuck in the line which acted like a little valve. That could happen too.

I assume you've already done a boost leak check. Before you start chasing things, also find out where the hissing noise is coming from. You've already found something strange, fix it before chasing other things. It could be from the EGR system or lines as well.

 

[chittychittybangbang]

EGR<->N75 can be swapped. I'm not sure if there's an internal difference. The car will run without the MAF because the car substitutes a default value. However, if power is unchanged with it plugged in vs. not plugged in, the MAF, MAF plug, or wiring is bad. This is because the default value is low compared to a working MAF. Use VCDS to confirm the MAF value that the computer sees.

 

[chittychittybangbang]

Ah... I am happy to know that you had a good resolution. I will update the FAQ to include this problem.

In the picture below, was it #7 that caused the problem?