An empty boat trailer, especially aluminum, will bounce and wiggle like crazy sometimes. I thought the same thing when i first towed an unloaded trailer. Dont worry about it!
Currently building a 28 foot Great Alaskan pilothouse boat!
My last boat was a 2008 Sea Hunt 220 Triton with Yamaha 250. Garmin 4212, Furuno FCV-620, GSD 22, Standard Horizon Matrix, XM Weather.