Hello Hubbabubba,
I figure you are smart enough to diagnose the noise problem from a computer, but for those who are not:
The noise generally comes from just a few pieces:
1. Cooling Fan of Power Supply.
2. Additional Cooling Fans in various places in the machine such as on the CPU.
3. Hard Disk Drive
4. CD / DVD Drive
5. Floppy Drives <--- IF you still have them. I have a few in various machines.
The order I listed is the order I believe to be most likely.
The first three are constants. The others obviously don't make noise if they are not spinning.
Electronics for the most part are pretty quiet but occasionally have a high pitched tone but that should be nearly inaudible with the case closed.
With the cover off, you should be able to hear much better where the noise is coming from.
There is another possibility which is kind of a combination: The noise might be vibration of the case caused by one of the motors described above or by a cable vibrating against the case. If this is the issue, you can generally slightly twist the case so that it doesn't vibrate or tie off the cables.
I use either small zip ties or plastic twist ties. Just make sure the pieces don't come loose.
If it is he Hard Disk Drive, then plan on acquiring a new one soon. They should not make much noise unless something is wrong such as a failing bearing.
Cooling Fans are not terribly hard to replace but with a modern machine, make sure the new one is really suited to your particular CPU.
I tend to wire in extra fans when I can if I suspect something is running too hot. The fans are the standard size muffin fans pulled out of discarded machines or power supplies. Old Apple cooling fans tend to be loud because they are run at varying speeds depending on temperature sensors. With a simple wired in setup, they won't have the speed control and always be running full blast.
I HAVE replaced the cooling fan in a power supply before but I recommend you don't do it yourself. The possibility of really screwing something up is pretty high.
At the moment, I know my old Pentium 233 has a power supply issue but I don't happen to have a spare side-switch AT Power Supply and those haven't been sold for a long time.
- Ivan.