1997 Geo Prizm has noise problem?

Geo has 200,000 on it and runs like a charm...do all proper maintenance etc....has low noise "grr" when it first starts up in drive and makes same noise louder when in park or especially reverse...no problem with engine performance or shifiting (automatic)..just notice a crack on the cover to manifold where the screw is...must old since rust on it...very small but dont think the screw could be placed back in ......someone mentioned replacing a "doughnut" where the exhaust connects to the manifold....you can hear the noise when i step on the gas.....after car runs it seems to quiet a bit....thanks! if this makes any sense to anyone!