Our mower is a Mountfield 1436H (one of the many clones from Castelgarden/GGP). I believe that the transmission is a Tuff Torq K46S. It's a 2003 model, and they're one of the manufacturers that claim the hydro is sealed for life with no maintenance required. Of course now I know that's not the case, but it means that for the last few years the machine's just been used and the transmission not checked other than for oil leaks.
My issue is that the machine suddenly started making a moaning, almost grinding noise from the transmission. The noise is continuous when the engine's running and the belt's in place, not just when it drives forward or back. It doesn't appear to affect driving power, but of course I'm not keen on running it while it makes this noise. I checked the oil level once I worked out how to access the reservoir, and the level could well have been low. Complete oil change hasn't helped the noise.
Any ideas for sensible checks before getting stuck in and dismantling the axle? My guess at the moment is that oil was low or something, and now there's been some mechanical damage internally. Maybe the top bearing, that takes the belt tension.
Thanks in advance,