2005 R52. 90k

So I'll try to do a good job describing this, and later today try to take a video.

There is a noticable vibration from the engine bay, specifically the supercharger pulley, but only in an rpm range just off idle.

For example, if the car sits at idle, there is no vibration, but if I blip the throttle slightly, it is visible and loud enough as well to be heard inside the car. It also (possibly, this is subjective) is more noticeable/worse if I lug the engine from a dead stop or slow roll.

It goes away as soon as the rpms rise above 1300ish (approximately, still testing this) so the range is really only a few hundred rpms.

​​​​​Pushing the clutch in changes nothing, it happens in or out, but turning off the AC causes the problem to go away. I want to say "completely" but I feel like it has occasionally still happened with no AC, but at significantly reduced intensity. Maybe. I can't reproduce it like that currently.

Supercharger was replaced at 60k with a brand new OEM unit under warranty, and has 30k in it. Sounds fine with a stethoscope and turns freely with the belt off.

Crank pulley was replaced with a fluid dampened unit from PCE (doesn't seem to be a known brand, got it from a tuner who sells on eBay) and the belt tensioner assembly was completely replaced because of a dampener failure. Idler pulley as well. I expected this to solve the problem, and it didn't. Same as before.

​​​​I worked through the threads and did a ton of reading but I'm stumped. The vibration of the nose of the SC worries me, but it's been like this for a while (it sounds a lot like a normal vibration that any car being lugged tends to do) and honestly, part of me thinks it may be normal.

But... It REALLY vibrates visibly. Like, worryingly so.

I would appreciate any ideas about where to go next.

Thanks!

 

Since turning the AC off visibly lower the issue, I would suspect the AC compressor clutch coil is going out. Read up on how to test it, I unfortunately cannot help much because I've never done so.
I guess a first test is to try to get your stethoscope there and listen if it comes from there. You could also put 12V on it directly to see if it engages/disengage correctly. Obviously the pulley should rotate freely when it's disengaged.

See that video to see where it is and how to change if needed:

 

So I tested this as well as I could, and used a scope. It doesn't seem to be coming from the AC, more that the tension on the belt changes when the AC is on and causes the vibration then? Still testing.