prolly the gasket around the PCV valve. cost is about $1-$1.5 from a dealer and you may as well replace the valve itself but dont get one from a dealer ($20) get a generic one from local auto parts store (<$8)

 

lol.. i always hated talking about this subject, since one time it started a 8 page thread lol.. but that is the PCV Valve

here is a tutorial:

http://www.lexls.com/tutorials/emission/pcv_valve.html

 

A thorough cleaning of the throttle body and a fuel injection treatment..the good kind using BG product about 80.00-125.00 ...the car will FLY after these are done..i guarantee you will notice the difference the minute you put your foot on the gas pedal.