Imac problems

Hello!
I don't know if this is an appropriate forum for asking about Imac problems, but if not, maybe someone can point me in the proper direction. My Imac (G4 800Mhz running OS 10.2) has just started to emit a non stop clicking sound from the speakers upon booting - in fact it sounds like a moped! Sometimes it stops when programs start up, sometimes not. It all started after making changes to the screen saver. I have it set on 5 minutes. When I came back after supper, the screen saver had not turned on. Very strange. Any help would be much appreciated. Is there any way of knowing if this is a software or hardware problem?
Thanks,
muzhok