Today I test drove a 2005 S40, 2.4i with 92k miles. During the short test drive I noticed a whistling or howling noise that seemed to be coming from behind or under the dash area. The sound was similar to when you blow across the top of a bottle. During the test drive the sound seemed to oscillate in volume.
When at a red light for a while it stopped, then started again when I started driving again. Once back at the lot it seemed to continue with constant volume at idle. I could not hear the noise outside the car or under the hood, just inside.
The noise was present with the a/c & fan on or off. I moved the vent deflectors around and that was not it.
I really like the car but did not buy it yet. I did some research here and it seems this may be the brake booster? If I go back is there any test to verify this, or any other likely thing to check for?

This would be my first Volvo. Thanks.

 

Considering you are looking at buying the car (vs repairing your own), the prudent thing to do would be to tell the owner to have a mechanic assess the problem. There's a few things that can create noises like this - vacuum pump, brake booster and some odd things in the power steering etc. Unless you can find the source while the car is parked (you can try using a hose as a listening tube), best to have a pro assess the issue (so you can factor the repair into your offer) or pass on the car.