I take care of a dozen or so trailers this size for you youth sailing program. Some are quiet and you forget the trailer is there others kinda rumble and vibrate the tow vehicle a couple will rattle you fillings out especially when empty. I'd change out the bearings or a least repack them, make sure the fasteners in the tounge are tight and all the other points in the frame and bunks are secure. Only other thing to do is make sure the alignment is correct but if the spring attachments are welded on nothing you can do there. To check the alignment take a measurement from each side of the axle to a point on the tounge, I like to use the inner ubolt on the axle to the center of the jaw on the coupler, doesn't matter what you use so long as it is the same on both sides. After that turn the radio up.
One more thing, take about 5 psi out of the tires see if that changes things the belts on the tires could be separating that'll make a similar vibration to a bearing. If the tires are more than five years old and you are making a long trip or going to be traveling more than 35 MPH replace them.
Now turn the radio up
