2000 Montana w/121,000 miles: beep-beep-beep coming from passenger side rear while in gear (it stops when in park)...no dash indicators of problem..what would cause this?

Do you mean while in reverse? That is a warning signal for people behind you to let them know you are backing up.

No..in anything but PARK. I am thinking that it is a warning that it thinks that the automatic side-door closer didn't finish closing the door (it did)..sounds like there's reports of this kind of thing happening.

you know, it could possibly be the power sliding door, they did have recalls for them for not properly latching in place.

it must be some rust on the contact on one of your door so that signal dont go to the computer as all the door are close!!clean it with some soft sand paper and put some white grease on it after cleaining it!!

I had that problem and Bud's answer is the solution

I thought it might have something to do with cargo balance. It happened to me once years ago when I was going on a trip fully loaded. I rearranged things and it stopped if I remember right. It is happening now. I let my daughter use the van to move some boxes. It didn't do it until she unloaded the car. My automatic door hasn't worked for years nor does the latch from outside. I have to open it from the inside. Ah, just checked the automatic door wants to work now but the broken latch keeps it from functioning. I opened it and closed it with the button and now the buzzing has stopped. So in conclusion it is the side door alarm and has nothing to do with load balance. Mystery solved.