Engine runs weird, plus strange random noise

[Mad Cow]

Usually my motor vibrates a little bit at idle, just enough to feel it. But sometimes it'll just run perfectly after I start it, no vibrations and it even feels smoother and more responsive. No codes apart from SAI incorrect flow because my pump is dying, currently unplugged. This happens seemingly randomly, about once ever 3-4 days, although it did happen both times I left it parked outside in the evening this past week, maybe related, maybe coincidence. When it runs well it runs well from the second I start it, nothing changes when it warms up. What could cause this? I'm out of ideas.

Now on to the noise, which has me completely baffled. It's coming from under the hood somewhere and it sounds almost like a dying electric motor trying to run, it's an incredibly loud screeching noise. It happens completely randomly, so far it's happened right after I started the car twice, once 5 minutes into a drive and once after a 15 minute drive. Generally the noise goes away after a couple of seconds so by the time I pop the hood to inspect it goes away. I thought it was my SAI pump but like I said before that's unplugged now and the noise still happens. It's not the aux or blower fan because I tried shutting off the climate control when it happened and it did nothing. Not my EFK either because I heard the same noise before I installed it, and it wouldn't be running right from startup anyway. Not sure if it's related but sometimes when I shut off the engine I hear a light rumbling sound from in front of the dash. What other electric motors do we have under the hood?

[Mad Cow]

Bump, still no closer to figuring it out.

[Crispy222]

The noise is probably the A/C belt. I think there is a 17mm hex to pull tension onto the belt and tighten it with another wrench at the same time. I'm not sure on the 2.8's.

My tensioner on my 1.8T had stripped threads and I couldn't keep the belt tight. Once I had everything apart, I put helicoils in the mount and made sure not to over tighten the bolts from there on out. The mount was Aluminum and easily stripped with M6 threads.

[Mad Cow]

2.8's have an automatic tensioner, plus my belt is almost new and the noise has continued after I shut the car off before. It's also nothing like belt noise, it's hard to explain but the best way I can describe it is a mix between a whine and a screech.

EDIT: Now that I think about it, it sounds a lot like when my last car had a plugged evap line and too much pressure built up in the gas tank, and I have heard vapours escaping when I open the gas cap sometimes. What part of the evap system is under the hood? I did my vacuum lines not too long ago but I don't remember anything about the actual components.