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.
I think changing power supplies is pretty easy. I recall only one screw holding it in place and disconnect a couple cables. Might cost about $ 100 for a new power supply. You might remove the power supply, open the power supply case and vacuum it out. I suspect there is nothing wrong with the power supply except dust on the fan.
Changing the PSU is easy, just remember and replace every cable like for like. There are four screws at the back holding the PSU in place. Occasionally there's another internal bracket but not likely. Keep the new cables away from any possible later movement into a fan blade. Most computers will run fine on a standard 350 watt ATX PSU. If you have multiple or high powered graphics cards etc., a 450 watt. You can get quiet PSU's check for user comments online before buying. Anything between $20 and $50 will be fine. I pay less than that for replacements I give to my customers and have never had a problem.
The thing to get is ball-bearing fans. That's usually the part that fails first in a PS and then the whole thing overheats and self-destructs (sometimes taking out the MB with it). On any computer I get, I always open up the PS and check to see if it's got a ball-bearing fan. If not, I'll replace all the fans with ball-bearing ones. Really helps with overall noise & vibration as well.