This mower has a hydo transmission. It has started making a load noise when moved into drive or reverse and will hardly move. Can it be a low fluid issue or what else?

Hi,
Your manual is available here:
Scotts Lawn Tractor Operator's Manual | ManualsOnline
And it will say it is sealed. This technically true, however it can be checked and added to if necessary. When the first L120s were failing, the factory had the dealer I worked for and others check them rather than pay to ship them back. It should someplace it leaked from to need fluid. Use good name brand 10W30 with an SH rating or better.
The cap #4 can be removed and a rule inserted to measure fluid height. Should be within 30mm of the top edge. It has to have expansion space.
Peace,
Clarke