Sounds like another dead tuff torq.Those are supposed to be sealed units in the earlier X300 series.
Transmission: Tuff Torq K58 Type: hydrostatic Gears: infinite forward and reverse Oil change: sealed unit, service not recommended
Did find this it may help.Not sure if your's will have this fluid level check hole but none of the earlier X300s have a drain hole so it must be removed from the tractor to drain it.
https://www.tufftorqservices.com/EnvEEdefault/FlatHTML/TechInfo/ttcoil/pdfs/cc%20K58%20MODELS%20CHECKING%20AND%20%20ADDING%20OIL-rev.pdf
Whining in a small hydrostatic like that is from cavitation either from Air in the system or excessively worn drive Motor.
There are alot of videos of people upgrading to the Tuff Torq K66 and K66Y in there earlier X300 series machines but your looking at $800 on ebay for a new one or $300 for a good used one.There are a couple sites that offer upgrade kits but there very very expencive.$1,800-$2,200.
It's recommended that the fluid be changed every 100 hours and internal filter (if equipped) every 200 hours.