Mystery sound...

Ok has anything happened to the car? Does it do it driving or sitting still? Does it change with roadspeed or rpms?

Few things to check
If you've taken a bottoming out might have knocked you manifolds loose and causing an exhaust leak the metal gaskets make a very strange sound when they first start to go.
Idler pulley like other have said
Major belt slippage
Throwout bearing starting to go
Intake leak somewhere. IE CAI, Plenum, Tb, ETC
Wheel bearing ( of course you'd only hear that going down the road)
Altenator going out ( I've seen some make all kinds of noise when they start to go)
exhaust leak
EGR leak


or maybe the Supercharger Fairies visited you. If so please send them to my place

Maybe the A/C compressor? Could be on its way to seizing...

i have the same noise, and i cant figure it out either... I am going to take a stab at a few occurances that might help decide if we are hunting the same problem or not.

my noise happens when its usually warm out, after the engine has heated up, and only at idle... once i get the RPMs passed 1k it goes away. the sound to me doesnt remind me of air anything, its kinda more along the lines of taking a martini glass, weting your finger with a lime, and running it around the edge of the glass making that reerberation sound.... but its loud.

as far as idler pully, i soaked all my pullies with PB blaster but never went away when the engine is on until i get the RPMs up passed 1k. it drops back down, and the noise comes back. its not an everyday noise, it just comes on, and after that it doesnt go away till it cools down.

just something to think about. mine kinda sounds like its on top of the engine... just cant seem to pinpoint it myself either... been living with it till this weekend when i can spend all day poking at various parts.

Torinalth