Really common with the 24 volt starters.
The contacts in the starter solenoid erode, so even though you get contact and some current, there's not enough current passing through the solenoid to keep it energised, so it bounces open. Once open, there's enough current to get it to re-engage, but not stay engaged. So you get click,click,click
Check
1st Contacts in the starter solenoid
2nd battery voltage under load ie starting. Both should read above 12 volts
3rd battery positive and starter ground leads and connections
4th 12-24 volt change over solenoid
I replaced contracts in mine with about $20 worth of generic parts from my local auto electrician