1996 Toyota Previa with approx. 150K miles.
It runs well for the most part, however when I am on the freeway, and around 50-60mph, I will get a
periodic THUNK when accelerating on slight inclines... sometimes even on level surfaces / lanes.

It can be pretty loud, sounding something like I just ran over a pot hole or good size speed bump.
I am assuming it is some type of transmission issue. It's doesn't seem to happen below 50mph
or at least I do not hear it. On a 30 minute journey down the freeway, I may experience it 2 or 3 times, always on an acceleration. It is not a pleasant sound. If I am careful with the gas pedal,
I can somewhat lessen the occurrences.

I will be seeing a mechanic about it at some point, but want to sound somewhat knowledgeable when I do.
What is going on inside the transmission that could be causing this, and how extensive of a repair might it be?

How should I present / describe it to the mechanic in "mechanic talk".

What is a reasonable amount or range that this should cost to repair?

Should I go to a regular mechanic, or start with a "transmission place"?

thanks...

i had this happen to me very annoying. check the spare tire under the van.CAREFULLY. even take it out because i laughed to when that was suggested to me but in the end they were right. seems they lossen.....