Strange Thumping Noise (sometimes)

So my 2021 Prius AWD just went over 500 miles. (I filled it up, got 65 mpg...whoop!) This is our 2nd Prius, my wife has a 2017 Prius 2.
Anyway, 2-3 times, in the morning when the car is cold, I've headed out to work with her and the gas engine turns on normally when I turn out onto the main street and I heard a strange thump thump a couple of times as the engine starts. I've never heard that in our 2017. I've been leaving the sound off in the mornings to see if she does it again but hasn't in the past few days.
Hopefully it's just something breaking in, has anybody else experienced this? I may put her in maintenance mode tomorrow morning and see if the gas engine makes that sound and record it for the Toyota Technician to listen to it.