Camry 2012+ :: SE - Rattling Or Clicking Noise From Front Passenger Seat While Driving?

For the last few months I noticed that there is a rattling or clicking noise that frequently comes from the front passenger seat area but only when no one is in that seat. If the seat is being weighed down by someone sitting there, there is no rattle or clicking noise. It generally occurs when I'm driving on bumpier or textured roads. It suspect that it's coming from something underneath the seat, like the lever that adjusts the seat forward and backward underneath it.

I brought the car to a local dealership and they took a look. After a few hours with them, the car was returned to me. They said they'd found that the lever that adjusts the seat forward/backward was not tightly screwed down all the way on one side, so they adjusted that to make it tight. They also found a coin on the rail underneath that seat and removed it too. However, I drove the car afterward and it is still making the noise.

I've searched this forum and already think I've ruled out the seat belt or the buckle as an issue. I put on the seat belt for the passenger seat and the noise still happens. It's really annoying when driving on a freeway!