The designs can vary from model to model so without a complete model number there is limited information we can provide.
Most systems use some form of cable or linkage to move a tensioner pulley assembly into the drive belt and tighten it so input is provided to the transmission to move the tractor. There might be some problem with a bearing or pulley not moving or spinning freely which is creating the noise and preventing the belt from tightening enough to provide proper input to the transmission.
If the belt looks okay and nothing seems to be slipping or out of place then the issue might be with the transmission itself. Some transmissions rely on the proper fluid level to create movement so if the level is low that could be a cause.
Or, there might be some issue with the brake sticking. Some brakes are inside the transmission while others are outside it, but any one of them could stick and create binding and drag which would create noise and reduce drive.
Or, possible the transmission itself has failed.
should have more specifics and troubleshooting tips for your particular model and design and reviewing the parts illustrations at the
site might also clarify how the parts work together.
Or, feel free to submit another question here with a complete model number and we will do our best to help further.