I don't have that trailer, but ultralights use a sandwiched floor system. 1/4 luan, foam insulation, another layer if 1/4" our so ply. If any of the adhesive stars to fail, you will get squeaks. If not that, then it is just a point that isn't fastened well enough to the frame.
As for the squirreliness while towing. Keep your speed under 65(that's all the tires are rated for) , and set your wdh according to the manual. After that, its trial and error. But yes, it is a big ole sale back there.
Joel and Teresa
2016 Sabre 34TBOK
2006 GMC 3500 CC LB DRW 4X4