Is the noise coming from around the curved exterior panel behind the drivers door? Here are my ideas of what to check out:
The first thing I'd look at is the storage drawers below the dinette seat. Check to see if something shifted, fell out of the drawer and is rolling around, or if one of the seat belts is flopping around.
Then next the generator compartment since it is directly below that area. That curved exterior panel would make a good echo camber.
Have you used your dinette slide out since the noise started? One of the electric motors could have come loose at the top of the slide out.
I'd also make sure nothing shifted in the over cab storage areas...
You might want to ask in
this thread as well as it is where most of the GTS owners hang out. There are some very good mods in it...