High pitched screeching noise!  Driving me nuts!

I've searched the questions and can't seem to find an answer after wading through several pages. BTW I'm new to MAC so need a "dummies" answer please. I have an iMac - 2007 and if have to keep the volume turned off at all times (unless I'm listening to music) because every time it makes a "normal" sound like sending mail - it screeches for about 15 seconds. My poor dog keeps shaking her head and I want to scream myself. It's done this for probably a year now, I've learned to live with it, but there has GOT to be a way to fix this? I have no clue where the noise comes from or how to get rid of it. Any idea would be appreciated!
I did upgrade to snow leopard, but that hasn't changed the problem at all (didn't think it would but there is always hope right?).
Thank You
Gayle

As you have upgraded to Snow Leopard, you must have an Intel-based iMac. This forum is for the older G4 iMacs with the articulated display arm and a dome-shaped base.
Suggest you repost in the Intel iMAc forums here:
http://discussions.apple.com/category.jspa?categoryID=189
Don't worry about misposting. The labeling for this forum is dismal and lots of people with your model are drawn here by the generic forum description; basically, all iMacs since the very first ones are "flat panel," so the wording was poorly chosen.

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    http://discussions.apple.com/thread.jspa?messageID=2831962#2831962
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    http://discussions.apple.com/thread.jspa?threadID=435160&start=0&tstart=0
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    iPod Photo: unhappy icon + warranty question
    http://discussions.apple.com/thread.jspa?messageID=2233746#2233746
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