Assuming you do not have a turbo it could be the flame trap.

This serves a similar function to a PCV valve. Very easy

to clean and service which should be done once a year. See:

http://volvospeed.com/Repair/ftrap.htm

Note that some service technicians pull out the part you have

to clean and do not replace it (mine did). They do this cause

it can clog and cause oil seal problems (rear main seal is a

common problem area).