1994 Pontiac Grand Am banging sound in engine when cold

1994 3.1L V6,automatic. When cold you start car and hear sveral banging sounds from engine compartment,there is no certain pattern if you rev up the engine as it idles down there is a load bang but it only happens until the engine starts to heat up and after about 2-3 minutes it goes away if you then rev up engine it idles down fine.Could it be some kind of exhaust-backpressure problem.....Please help....My email is plsanner@mchsi.com.....Thank you.....Phil