My MCS started making a low grinding noise on the way to work at 75. Half way there I noticed it goes away or gets less at higher RPM but gets louder at lower RPM. On the way home it wasn't as bad until half way there. That's when it turned into a higher pitched grinding noise with the occasional clunking. Released the tension off the tensioner & started it up & it was quiet so it has to be either one of the pulleys or the supercharger itself. But while driving the supercharger was boosting with no problems just the grinding noise. Is there a way to figure out what's going on? Tried using the screwdriver to ear trick but was only able to do it with the tensioner bracket that's connected to supercharger & it's definitely in that general area.
My first thought is bad supercharger or bad tensioner/belt. If the tensioner and belt are more than a year old I would replace them along with the idler pulley. Also check the crank pulley if its the stock one they are know to fail. If the rubber on it has cracks replace it. You could listen to the supercharger with it idling. I use a long screwdriver as a stethoscope. The gears in the rear of the supercharge that run the water pump may have run dry and are failing.
I've also seen quite a few tensioner and idler pulley failures, rule them out first by taking off the belt, you can spin those and tell if one is failing. Definitely check the crank pulley too.....you can usually tell if one of those is failing just by looking at the rubber section to see if it's damaged
things that could make noise and then not when you take the belt off: There are small gears in the supercharger that make a reduction transmission to drive the water pump. If the oil in there hasn't been serviced in ages, the teeth will go out and start sounding nasty. Sound is totally RPM dependent on that one. Don't think it would be a low grinding noise, but much more high pitched. It could be the water pump itself, but that is on the other end of the SC, so your screwdriver sound check should be able to clearly ID that as the issue. The gears that drive the SC are on that end as well, so same check. Those pumps don't have fancy bearings that could fail like wheel bearing and start rumbling. Again, don't think that is where it's at. Could be AC pump clutch - disconnected from drive when you loosen the belt Could be alternator - also disconnected from drive when you loosen the belt. Don't think the crank pulley would make that sound. If it was that badly damaged to make a low grinding sound, you'd likely smell burnt rubber and your belt may be showing edge wear from not properly tracking on all the pulleys any longer. Idler pulley is cheap - bearings when they go can make all sorts of noises. If you haven't replaced any of those parts in the last 20k miles, just do it. I'd replace those parts at least every 30k as preventive maintenance. They are cheap on Amazon or at Rock Auto. Check if the SC pulley is tight (some have 4 bolts pulling it onto a cone press fit. Then throw on a new belt and check the crank pulley while you have the inner fender liner off. If that's bad, get an aftermarket pulley to cure that issue once and for all. Just guessing, based on the noise being closer to the belt side than the water pump side of the super charger, I'd check AC clutch first (does noise go away when engaged?), then replace idler pulley and tensioner. If that doesn't cure it, have alternator looked at or just replace it if the car has 100+k miles on it.