It's most likely an intake manifold gasket leak, very common on the GM 3.1 and 3.4 engines. If you have mechanical ability, it is not a job beyond a DIY, but it involves taking the entire top of the engine apart and a couple items some people don't have (in/lb calibrated torque wrench, loctite, etc...) IT would be a weekend project for your first time (I've done quite a few and have it down to about 5 hours). Organization and cleanliness are your two best friends if you attempt this yourself. If you're not too sure of your skills, it's best to leave to a mechanic. Shop around, as the price for this job varies from $450-$900. Make sure wherever you take it uses the improved design metal-framed gaskets, not the junk plastic-framed oem design (unless you want to do this again in a year or two).