I have an aluminum trailer under my 18ft Mako. On my last trip I noticed a course vibration between 40 and 50 mph; smooth otherwise. Lugs are tight and hubs and bearings seem fine and are well lubed(no extra heat that I could notice). Rig spends equal time in fresh and salt water. Wheels have no weights on them inside or out so I'm not exactly sure where to start. Any thoughts would be appreciated.