what's causing a clicking sound once in awhile inside the iMac ?

The only moving part in an iMac is its exhaust fan. Some iMacs also have a mechanical hard disk drive. If you are hearing it only once in a while it's probably the latter.

I wouldn't worry about it. Anything can fail at any time, and everything will fail eventually. Just be sure to back up your Mac, for that reason. To learn how to use Time Machine please read Back up your files with Time Machine on Mac.

Noises from a mechanical hard disk drive are normal. If it's broken it's not as though you could do anything about it on your own anyway. It would take at least an experienced set of ears to discern what that noise might be. If you are so motivated, you could always contact Apple Support to get their opinion: Get Support.