Sounds like a bad power supply or bad logic board. Also it could be anything that's hooked up to the Mac. I have a Mouse that will cause a Mac LC to do this very thing, and when I unplug it, everything is fine. (It must have a short circuit.)

I would first try removing everything possible, i.e., remove any expansion card (ethernet / modem / video / IIe, etc) and also only hook up the power on the back, no keyboard, SCSI, etc. See if that makes a startup sound that way.

Even without a PRAM battery, it should make a startup sound.