Never heard a slipping clutch make any sound. For a gear drive, only one thing will allow the engine to maintain RPM but no forward motion. You need to get that clutch adjusted NOW before the slipping burns it up. Hopefully it is only an adjustment needed and not a new clutch plate. With a shuttle shift there is also a hydraulic clutch action that briefly depresses the clutch plate when the shuttle shift lever is moved, it could be that the hydraulic clutch is not disengaging correctly also. A trip to the dealer is recommended for this.