Thanks for all the suggestions.
UPDATE:
Well, I had the cover off three days in a row and, of course it stopped making the noise. You know...like asking your mechanic to find that mysterious squeal.
Anyway, it happened at startup this AM so I put my ear down and then reached around the the back [it's under a desk] and pressed around the power cord.
The noise changed drastically, louder softer, etc. I could feel the vibration big time and then felt it dissipate as the pc completely booted up. Once up and running, no vibration at all back there. Wow, I'm getting excited ;-)
The fan I replaced was pretty big...had a real big green plastic hood over it [it's a DELL] and it was brand new. I call that the "main fan". Not sure if that's the power supply fan or not, but it was brand new.
Is it a big deal to change the power supply? I'll just do that. I love this machine and don't mind putting some more $ into it.
Thanks again,
V.