2001 xterra; v6; 155k miles

After I changed my oil, I noticed a clicking sound coming from the top half of the engine. I could only hear it with the hood up. Thinking it might be ignition related, felt each plug wire, but couldn't feel anything. Any suggestions?

Another problem (possibly related/contributing) - stumbling in 1st and 2nd gear, only after engine is good and warmed up. Doesn't happen at all with cold engine. Could this be a fuel filter issue, when engine is cold doesn't happen because of the xtra fuel being pumped? Was thinking it could be my clutch starting to go - but that wouldn't explain why it only happens with a warmed engine. Any ideas? When I changed the oil, I added some lucas upper cyl lube/injector cleaner and the problem almost went away, but not quite.