1974 144 4-speed w/ overdrive

So, I haven't had this car for too incredibly long (a month or so), and I was just getting ready to flush the transmission fluid ovrhauling all the small stuff as quickly as time allows). On my way to the auto parts store, I started hearing a really loud clunking. The car was still driving fine, but the noise was scary. I was home in a couple miles, and upon first inspection of the transmission fluid, the case was empty as far down as I could put my finger.

What does the noise sound like?
Exactly like one of those gear driven toys when you wind it up too much, and it starts trying to spin the gears too fast. You get that "pizzz-chik-chik-pizzz-chik" sound like it's skipping teeth.

When does it do it?
In neutral, whenever I rev the engine above about 1,500rpms. And in all gears most of the time. It's intermittent, but constantly intermittent (does that make sense?). You hear it less in the higher gears.

I noted when the sound is made after filling the transmission with SAE30 oil per the Chilton manual. Am I looking at replacing the transmission, or could it be something else? I just thought I'd hear some ideas before I started tearing things apart. Like I said, the engine runs fine, and the transmission shifts fine too.

Thanks for any help.

Ian

P.S. I did try searching first, but I find it rather difficult to use this site's search engine.