ONAN generator no start

Are the house batteries charged? The Genset uses the house battery for starting. You may need to plug into power or use a charger to bring the house battery up. It may work to hold the aux sw on the dash while starting the genset. (If the battery has enough power to close the relay)

From your post it sounds like the engine is not cranking. Will it crank if your push the combiner button on the dash? Clean all battery connections first. Does it turn over? If you have a clamp-on amp meter check to see if amps are getting to the starter. If you don't have an ammeter back your tow car and connect jumper cables directly to the genset. Will it turn over now?

If you're hearing a rapid clicking (not all that loud) it could just be the fuel pump not getting fuel. Make sure you have enough gas in the tank.

If it's not cranking, check connections as mentioned above.

Check all your connections between the battery and the generator, both grounds and positive wires. While a bad starter can certainly cause the clicking you're hearing, a poor connection and the consequent votlage drop at the generator when cranking can do the same thing. A bad starter solenoid in the generator, if it's separate from the starter motor as I think most are, could also be another cause.

That "machine gun" sound suggests a failed starter gear that's failing to engage with the flywheel. Depending on the model, it may be possible to replace the gear mechanism, or it may require replacing the entire starter. If the unit is older, a complete starter would likely be the better choice anyway.

Did you check the circuit breakers on the generator? Remove the cover and (at least on my generator), you will find rocker switches for the circuit breakers. I've had mine trip and the first time it happened, I would press the start button on the dash, and all I would get was the clicking sound of the generator trying to start. Once I removed the cover and flipped the circuit breaker switches, the problem was solved.