My prior thump sound posting(different thread) comparing my two odyssey appears to have vanished.
The conclusion made before was that my '99 did not have the thump noise while my '04 had it.
Well, I have a correction -- they both do it, but there is a solution.
What I have discovered is that if almost anything is stored under the open third row seat the noise vanishes.
I came to this conclusion after beginning to move the stuff stored in the '99 to the '04 and I began noticing the thump in the '99.
I still have the '04 pretty empty, but I have noticed that the Honda cargo mat over the folder down third row helps. And I have discovered that storing the cargo mat rolled up under the third row seat makes the thump pretty much go away.
So a mat or kids blanks/extra clothes stuffed under the third row seat solves the problem.