I posted this on V-Spy previously, but I couldn't remember sharing it here. So please ignore if this is a repeat.

My T5 engine sometimes made a strange whistle-like sound. Sometimes soft, sometimes annoyingly loud, usually at idle after warm-up. Last Friday my dealer replaced the piston cooling valve (parts listed below). If you want more detail, look for my thread on VolvoSpy and read what "Volvo Tech" (appropriate name) has to say.

Big improvement! The noise is still there (sometimes), but the volume is so soft that only I can hear it only because I know its there (its a "If Hellen Keller fell in the woods, would she make any noise?" kinda thing). Anyway, I heard that the more recent T5's (mine was made in 3/00) have the updated part, earlier ones need minor surgery.

Valve - Part # 31104764-1

Gasket - Part # 11998-2

Good Luck!