OS 10.6.8 late 2009 imac. Yesterday and twice this morning, after booting up, the desktop screen went black for a few seconds and then the desktop became normal. I have just rebooted and it never reoccurred. Is this an indication of something I should know about? Secondly, for the first time I could hear the fan blowing upon boot-up so I rebooted and again the fan was very audible but after about 2 minutes, it was inaudible as usual. I realize the fan is is for cooling purposes but it must run at a very slow speed for me not to know it is actually running. Am I reading too much into this? jaybass

As far as the fan speed issue, resetting SMC may cure it (it did for me). I can't answer the question about the display.

Try running Apple's Hardware Diagnostics (I think below is the procedure for 10.6.8):

"Restart your Mac, holding down the D key while the Mac restarts.
After your Mac restarts, you should see the Apple Hardware Test chooser screen. If you don’t, Apple Hardware Test may not be available on your Mac. You may be able to start Apple Hardware Test from the Internet. Reconnect your Mac to the network, then restart your Mac while holding down both the Option and D keys."

Ira, I did the Apple Hardware Test and it is okay. Thank you for that.

Jon, The fan is behaving just now but if it starts to run fast, I now know what to do. Thanks. jaybass