Instead of pulley's, or belts, it sounds to me more like the pump on that side may be partially deadheading somewhat against the return side, like the pressure relief is not working properly. In other words, that pump is continually pumping oil, and returning to the reservoir, until you divert that oil with the forward/reverse lever, relieving that pressure.
I'd first have to check the pressure relief valve, and direction control valve, and see if they are working properly. It could be a sticky relief valve, or something stuck behind the relief valve, not letting it open fully, and partially deadheading the pump. Possible worn o-rings on the directional valve, trying to drive the drive motors in both directions, until you relieve the pressure, by moving forward, or reverse.
Here's a link to the Case/IH online parts store. I only went as far as showing all 3 models. You can look at each of them, and drill down, and figure out which model you have, then trace the hydraulic system for that model to locate the valves. There's more than likely an exploded view of the valves. They may even have o-ring kits to rebuild them.
Official Case IH Online Parts Store and Case IH Catalog for Best Parts Search and Best Parts Look Up. Case IH Parts for Case IH Tractors, Case IH Combines, Case IH Tractors, Case IH Manuals, Case IH Parts
