OK, thanks.
Depending on the exact model and configuration of your iMac, it could be a few things:
-Lower left - fan
-Upper left - power supply
-Left center to middle center - hard drive
You could always take it in to an Apple Store and have full hardware diagnostics run on it. It could be foreign material in the fan, hard drive starting to fail or intensely accessing data, power supply heating up and not venting properly, etc. Hard to tell without being able to hear it in person and run hardware diagnostics.
If you aren't experiencing any freezing/powering off/etc., then it is probably more of an annoyance than a hardware issue.
C