Single thunk very occasionally just after starting

Had a weird experience with my Nissan Altima (2003) in June that has repeated very occasionally since then. Immediately after starting, there is a single loud thunk. It sounds exactly like a bird flying into the rear left window. In fact, the first time I heard it I thought that’s what had happened. Since it has repeated – although very occasionally, as in a handful of times – I’m concerned that it’s a problem that needs to be addressed. The car has about 100K miles on it and is otherwise in very good condition. Can anyone shed light on what the likely issue might be? (I recently moved to a new area and I don’t have a trusted mechanic to ask.)

Immediately after the engine is started or immediately after you start moving?

Assuming the first, how soon? Does it happen with the transmission in neutral? or park?

Assuming right after you start moving, and the longer it sits or the more rain etc. the more likely. Do you use your parking brake? My guess is a rusting or sticking brake issue.

If the noise happens when you first start moving (rather than when the car is sitting still after starting the engine), then I would suspect that you are just hearing the ABS going through its normal self-test, which is supposed to take place after a cold start.

So…can you clarify if the noise is heard while sitting still or after you have moved the car a few feet?

The noise you hear might be the purge valve in the EVAP system opening when the engine is started.

In your vehicle, the gas tank, carbon canister, and all the EVAP valves are located under the rear seat.

When you start the engine, the computer determines if and when the purge valve should open to draw the fuel vapors from the carbon canister into the engine. And that might be the noise you hear.