Sound: Pebble in a soda can??

So, I've got a sound coming from passenger's side rear wheel that sounds like a pebble rattling around in a soda can. Only does it between 1 -10 mph, sound speeds up to 20 and then it goes away. Comes back as I slow down to stop. First I thought maybe a wheel bearing, but no screech, whine, or grinding noise -- Applying a little Occam's razor here - do I have a pebble lodged in the wheel/brake assembly somewhere or is this something more serious like loose brake calipers or something else?

Tried to look with a flashlight but couldn't see anything that looked to my inexperienced eyes to be out of place.

Also, applying the brakes doesn't stop the sound.

Anybody know what this could be?

 

Had a similar type rattle when I didn't have the second row seat securely latched.

 

Had that sound the day we left the dealer with our Sienna (although I don't remember if we had to be driving or if we could make it happen while just revving the engine slightly). Turned out it was a bad catalytic converter (something loose inside).

 

Yeah, this just showed up in the past few days or so. Just noticed it today because I pulled out of the garage with my windows down. I definitely think it's outside in the rear passenger wheel, but I'll check the seat RumHam. THX -- It definitely sounds like something rattling around inside that cover/shield over the brakes -- but damn if I can't see it.

 

just found a video about Toyota TPMS sensor that came off inside the tire -- sounds like it may be the culprit.

https://www.youtube.com/watch?v=rff7PmK4W4M

 

UPDATE: Well, found out the issue. It wasn't the TPMS sensor. It was a tire weight that had come loose that was knocking around the inside of the wheel.