"bottle of pills" rattling sound

I had to drive my wife to SEATAC this morning. I would normally drive my car (Camry), but my trunk is a mess with muddy tire chains and the back seat is a mess with muddy dog fur, etc.

So, we took her Prius. Heading back from the airport on Interstate 5, I notice a very familiar rattling sound, which is not unlike a half-full plastic bottle of pills being shaken.

This is the EXACT same sound I have heard in my Camry for at least a year. But, this morning in the Prius, I am able narrow it down. It seems to be coming from behind the plastic that covers the interior body at the upper driver side seat belt mount.

I had not been able to pin down the location of the Camry sound, but it is certainly coming from the same "general" area of behind the driver on the left side.

I'll do more listening in the Camry and see what I think, but I am fairly certain the seat belt mount area is the location of the noise. Wife has never commented on it, but she is partially deaf on her left side, so doubt she ever hears it.

It is not a constant, it seems to come and go.

My question, anyone else experience this? Seat belts seem to work. I really hate to think of trying to take apart any of that area. I did it once with a Datsun in the 1980s and was never able to get it back together properly.