Why is my generator "clunking"?

[baldwinh]

My 8kw Onan Diesel generator has been working fine, when I push the pre-heat switch it goes "tickety-tickety" and after about 15 seconds fires right up.
This week while running it, it started to emit more of a "clunking" sound, very rythmic and regular (and pretty loud). I shut it down, today checked the oil level (clean and full) and when I pushed the Preheat button again to try staring, this time it gave off the "clunking" sound instead of the quiet purr of the normal "tickety-tickety".

Any ideas on what might be the problem? I didn't leave the preheat button pushed long enough to try to start it for fear the clunking was ruining something.

Thanks in advance for any ideas / input

[MEXICOWANDERER]

Clunking while cranking could easily be the starter drive which has a integral clutch. Pray it's not shearing teeth off the ring gear.

[DrewE]

If it's happening while it's preheating, it's not something in the engine mechanicals that are not yet moving. My guess would be an electric fuel pump. Disclaimer: I don't know anything specific about the particular generator model.