Transmission or Clutch gone out?

I was cruising down the highway the other night when all of a sudden my motor reved up to 4 rpms and then the truck coasted to a stop. The engine turns over, but there is a terrible noise coming from under the truck. 5 speed manual transmission. 1995 GMC Sierra. Towing it to a mechanic today. Could it be the transmission/clutch went out??