I have a 2001 s40, and I've heard a rattle kind of a sound coming from somewhere in the engine compartment for the past few years now. It only happens AFTER the car has been warmed up for a little while, and only while I am accelerating. You hear it the most in 1st or 2nd gear when the RPMS are revving up just before the gears shift. It sounds like something spinning with metal parts in it, but I can't figure it out. Any ideas? Could it be something with the turbo??? Even with the noise for the last few years, the engine runs like a champ.

 

May be a pulley to even worse a timing belt roller. Could also be something like your alternator or some other item connect to a drive belt. You said it was linked to rpms.