I just purchased a boat with a 1989 galvanized trailer. The trailer is made up of hollow rectangular beams. When I jack up the trailer and rotate the tires the trailer makes a rumbling sound. I have drive 1/2 hour with the boat on the trailer and there is no heat on the hubs and then jacked up the tires move very freely. What is causing this noise? Could it be the bearings? Also, is a slight amount of dry cracking on boat tires normal and ok? Thanks, it's my first boat in 20 years (since I was a teenager), and I didnt' pay attention to things back then