Got a weird one here. I've got a hissing noise that has slowly gotten louder over the past week or so. The best way to describe it is it sounds like I would expect the the expansion tank to sound if the car was overheating and spitting coolant out the seams at high pressure but low flow....but its not . I barely noticed it at first, but its definitely increased in volume. It's there as soon as the car is started and very consistent at idle. When driving with the car in gear but no throttle, or the car in neutral but idling it is at full volume, but as I apply throttle and approach neutral vacuum, the noise disappears.

Its definitely coming from the supercharger or something attached directly to it per my screwdriver stethoscope. As I move my head towards the back of the engine bay the noise is lessened, and its at full volume looking straight at the supercharger body, but it really doesn't sound mechanical. And if it WAS mechanical, I can't see any reason for it to be tied to vacuum. In my experience I've never really observed a boost/vacuum leak to be audible (above a very slight hiss with ones ear at the source). The car doesn't feel down on power, and to the best of my ability I haven't been able to observe the noise coming back under boost. It only appears to happen under vacuum, and is directly proportional to the amount of vacuum. No CEL is present. Any thoughts before I take everything apart?

Anyone have any thoughts? My plan is to pull the SC and check everything out, but it got an oil change ~7k miles ago. I did NOT replace the water pump. I should emphasize the volume, it seems too loud to simply be air being sucked in like a small vacuum leak. It's closer to using an air gun attached to a compressor.

 

A gasket or tubing has come off or split. The blow off valve could have gone bad also.
Also the intercooler rubber seals on both ends may have blown off or cracked.

 

Possible you cracked the s/c inlet tube when you did your recent s/c oil change?
If it's "airgun" loud, I would think you could hone in on the source fairly precisely...

Upgrade your screwdriver stethoscope to an actual one? Pretty cheap @ Harbor Freight.
I made one with a 10" length of 1/4" copper tubing inserted into a 36" length of 1/4" I.D. clear plastic tubing.

 

Check this tube​​​​​​​

 

I would start with intercooler rubber seals and look for split or loose hoses. Rubber rots with time.

 

Finally got a chance to diagnose with a friend. I had him tap the gas while I hunted around the engine bay. I could tell roughly where (supercharger) but not exactly where or what it was. It was so loud I found it hard to believe it wasn't mechanical. Being a decent backyard mechanic, I grabbed a water bottle and started spraying likely vacuum leak areas, thinking the noise would change if water got sucked into the leak. Sure enough, I was able to isolate it to the supercharger inlet gasket (which was replaced 7k miles ago). I took everything apart yesterday morning and found it had split at the radius nearest the block. Not sure what would cause a rubber gasket to crack like that, but have a new one and a spare on the way. Thanks for the help everyone. I'm assuming the increase in noise is due to the supercharger pulling air through and the leak being very small.