Onan generator doesn't start

[regbl10]

It's a model 5BGMFA26105F in '97 Southwind, which hasn't been used
for 18 months. When I press the starter, the engine doesn't turn over,
but it sounds like something is turning, making a chattering sound.

[Skip426]

First thing would be to make sure the gen set is getting full battery voltage.
Next would be to get as close to the gen set as possible while someone works the controls ( if you have on set starter button , you can work from there) . If the starter winds up or grinds and the gen set won't turn over, starter removal is required ; possible bad starter drive ; and manually turning the gen set over using a pry bar though the starter hole to the flywheel, to make sure the motor isn't seized.