After I start my Cummins, from a few seconds to a couple of minutes after start, there is a single 'thunk' sound. It happens just once and only when it is cold (doesn't happen after after I turn the engine off then on again, or even within a half hour). It's not predictable when it specifically happens. Usually it just happens a few seconds after start, but this morning I let it idle for 5 minutes and then it happened when I shifted from park into reverse.
I had someone mention that it might be the starter bendix. What other possibilities might there be?

 

just as a question sometimes the clutch kicking in for your air conditioning compressor makes a clunk type noise just wondering. where is the noise coming from? engine area or underneath try turning off ac and no defrost and see what happens