i have a 2010 s40 with 36k miles. there is a musty and even burnt smell coming from the vents. it smells musty with the vents blowing without the AC and a burnt smell when i have climate control on. seems to smell when cold goes off when it reaches temp. could it be the cabin filter and where is it. i cant find it.

 

Assuming Volvo didn't change anything from the 2005+ S40's (they're all based on the P1 chassis) the cabin air filter is be behind the center console. The access to it is from the driver side footwell.
You also have to remove the gas pedal to get to the filter.

Here's a wright up...
Swedespeed Forums - Cabin air filter writeup

 

thanks Mat, ill look there. any idea if this could be the cause of musty smell?

 

Not a problem. Don't know for sure but that'd be the first place I'd check.

 

It very well could be the source. My V50 starts to smell a little musty when it needs a new cabin filter.
Stan