Loud CLUNK on accelerating from stop

My 2011 Prius occasionally clunks loudly when starting from a complete stop. It feels as though something is stuck and then releases the second I step on the gas pedal, and I think the sound is coming from the rear of the car. It doesn't matter how long or short my stop is at an intersection, and there's no rhyme or reason as to when the CLUNK happens. These are definitely not hard accelerations, either.

I notice this issue when the car is being driven a long distance. It never happens on short trips.

Any ideas as to what might be going on?

For what it's worth, I have the car regularly serviced by my local dealer, but last year it look a lot of time and money for its service department to figure out that a "whump whump" sound was being caused by an unevenly worn tire, even though the tires were regularly rotated. (They replaced brakes and rotors first.)

Thanks for any help!