[Jackthemower//]

Hiya
When accelerating the mower there is a sharp short grind/vibration which sounds like it’s under the seat of the mower, this stops when the mower catches up to speed. For example when tapping the forward there is a quick grind. Also it struggles to reverse up any amount of incline.
Any suggestions ? (I think it’s the pulley on the trans…)

 

[rwmeyer]

I'm not familiar with the X300, but it sounds like your transmission is moving under hard acceleration. Look underneath for any loose or missing fasteners and put a wrench to all trans to tractor fasteners. You also may(??) be able to jack tractor, put blocks under rear axle to keep tires from touching the ground. Then jack tractor again, by jacking the frame, not the trans. As SOON as the frame starts to lift, the tires should too. Bob

 

[X300owner]

How many hours on the X300? Have you ever changed the oil in the trans? Here is a post about changing the oil in the K46 in the X300 which would be a great place to start for about $25.
K46 oil change with pictures
The pulley is another good idea and if you pull the trans checking it would be easy.

 

[Frogmore]

Also it struggles to reverse up any amount of incline

Best case, the tensioner and/or pulley on the transaxle need some care.
Worst case, the internals in the transaxle are worn. Changing the oil might help, but more likely, you will need to repair or replace the center section of the transaxle (pump and motor).

This sounds like the classic case of a worn K46.

 

[wally2q]

could also be worn/stripped splines on the transmission-input pulley... under surged load, it skips a few teeth....
bottom line - you need to inspect well before serious damage occurs.....
for example - if the transmission mounting is loose, the transmission case begins to torque under load, without support from the tractor-frame, and that could cause a crack to he aluminum transmission housing....

 

[joneill4]

Wally gets my vote with the stripped splines on the input pulley. It is a wearable part meant to spare the input shaft. Using it while it is skipping can damage the input shaft.

 

[Alien5044]

3rd support of the input spline stripping. Mostly due to the timing and description of the noise.