My 2005 s40 T5 just started making a whirring noise. Accelerating seems to stop it. It begins once the engine is warmed up. I can't find anything on the internet about the noise stopping when I accelerate. Any Ideas?

 

Does it sound like a fan spinning? We might have the same noise, but my car is a 1.9T. The noise occurs during low RPMs. When I stop at a stoplight, a few seconds later the noise kicks in and then goes away when I accelerate. Does this sound like your problem?

 

Does it stop if you open the oil fill cap? If so then it is very likely the crank case breather box.

http://www.ipdusa.com/Volvo-V50/Engi...-409-415-4271/

This is a common problem and Volvo has extended the warranty for this item. Check with Volvo Service.
Stan