2021 Limited CrewMax - Rattle coming from underneath

A rattle has started to develop under the truck recently...can hear it when going over bumps. Can't seem to pinpoint if it's front or read, if it's a suspension thing, or a loose protective plate, etc. I have RCI rock sliders on but these have been confirmed tightened. Ive taken it in to two different Toyota dealerships - one told me it was the spare tire being loose but obviously if it was that they would've tightened and therefore it would not still rattle. The other service manager said that it was my spray-in bed liner. I joke not. Anyway, about to take it in to a third dealer. I see there are a few posts on this site stating 2021 was a year of issues for the Tundra. Has anyone else experienced this?

 

Update to this - At the time, I was told by local Toyota mechanic it was my brake line hitting the frame and that I needed a small plastic clip. Worked for a while, now the sound is back but the clip is in place. Can't seem to get anyone to find the problem.

 

You check the obvious like sway bar and links, shocks, etc?

If the clip worked for a while, I’d look at that area a little more.

 

OK, will do. Thanks.