Why is my Mac Pro Beeping Randomly?

I looked, and I have found the other threads that address this beeping issue, but they don't seem to apply to what's happening with my Mac.

It beeps just once, but it is doing this all day at various intervals - like every half-hour to an hour. It's happening randomly, not when I'm doing anything specifically.

I've run disk utility and verify disk and looked at that s.m.a.r.t. thing (that was suggested in another thread) and that all checks out okay.

Thanks in advance for any ideas or suggestions as to what might be happening.

p.s. My Mac Pro is older (2007) and I'm running Snow Leopard 10.6.8