PowerMac G5 Quad Making "boop" sound

What is going on? For no apparent reason, this computer decided to start making random "booping" sounds this evening. There is no error, it's not the mail, it even does it when I have my external speakers turned off. Anyone else have this going on? This is the first time it has happened and it's been going on all evening. It's making me crazy. I bought it new in February.

Help.

-Lela

I do not have a CD in the drive now. I was using them all day though, as I was working with photos. Yes, it still does it after restarting.

-Lela

I have tried to notice what I'm doing when it happens. I can't associate it with any mouse commands or opening and closing of windows or programs or anything else.
And it doesn't sound like any of the built-in sound choices. As I said in the title, it's best described as a "boop."

I've been very busy on the computer all the day long. It has never napped today - although I'm not at all sure I understand
turning off "napping." I've read some other complaints about using the CHUD tools for that, but their problems did not sound like
the one I'm having.

-Lela

Well, that sounds like it's for "beeping" G5s - what about ours
that "boop?"

Okay, I'll do some searches.
Thanks.