1993 Geo Prizm 1993 Prizm (1.8 lt) misfires when warmed up

Noticed a whirring noise from the engine (sounded metallic - like a loose part vibrating at a very high frequency). Sound appears to be internal. Knew something bad was happening and planned to get it in for an engine check. Otherwise engine was operating OK. Problem developed a misfire symptom that can't be ignored. When car is started, seems to idle OK - no misfire. Once engine begins warming up, misfire begins. I tried new cap/rotor, but not getting off that easy. I haven't checked the plug wires (they are just 1 year old), but seems like it wouldn't be tied to engine temp. I'm thinking mechanical issue with distributer, but they cost ~$200 for a a good rebuilt and hate to experiment at those prices. Whirring noise frequency does seem to be tied to RPM (more RPMs faster whirring) and seems to stop for a microsec at when the misfire occurs (but that could also be my imagination). Other possibilities running thru my head are timing belt idler or valve/lifter.