Transmission & Differential - Clutch Slipping at launch?

On my 91 Si when launching from a stop, I sometimes here a low pitched whine, almost a howl. It sounds kind of like a slipping belt, but at a much lower pitch. This is happening more and more frequently. It doesn't have to be a quick launch, but it seems to happen more regularly on quick starts.

Any ideas or suggestions on this one?

Thanks.