When the engine is running, take a long screwdriver and place it against the s/c snout. Take the other end (handle) and place it against your palm while your ear is resting against the outside of your hand. You will be surprised what you can hear using the screwdriver as a conduit.
My guess: bad supercharger pulley.
When mine was worn out (120k miles), it sounded like rocks were tumbling inside my engine. I could only hear at idle. It was so bad that I was sure the needle bearings were trashed. When I cracked it open (took the lobes out and everything), the only thing that was bad was the pulley.
The car has been whisper quiet ever since replacement.
