> I find the slight muffling to be interesting.  Might be an artifact but to
> me it doesn't have the clear ring of two heavy parts tapping together.  More
> like something heavyish and some sheet metal.
>
>
This is what it sounds like to me too.  Like something bouncing off the
bottom of the van, for example.  But I've spent a lot of time under there
now looking for things, and I find nothing.  And with such a prominent
noise, I would expect it to be something that is obviously loose when you
push on it.