Do you think this is the beginning of a transmission problem?

2004 Honda Odyssey. 150k
Runs great. Pretty decently maintained. Bought at Carmax 99k miles.
No odd noises. Good tires. Clean oil. Runs great. No

Accelerating from standstill, it mysteriously bumps... something in the engine goes "thump" and you can feel it a bit (not just hear it) somewhere around 1st gear. It happens in reverse as well. If you accelerate slowly you can usually avoid the thump. No odd whining noises, or anything that grabs the attention other than the regular noise of a smooth running engine.

... After that single the car drives perfectly.
... it only started in the last few hundred miles.

 

Yea, that is the early sign of the tranny going south. My '03 did that and ran perfect for a few thousand miles and randomly down shift while doing the highway speed. it was like someone randomly slammed the brake, let go and slammed the brake and let go. If you check the tranny fluid, it probably not red anymore.