You could jack up one side of the trailer, and spin the wheels by hand to try and locate the noise. How old is the trailer? I just had my 2010 serviced... brakes inspected, cleaned, and adjusted, and the wheels inspected, cleaned, greased and seals replaced plus tires inspected and balanced for $193 at a local Truck/Auto Repair shop. It was well worth the money.
Mike, Sue, Sissy and Little Man
2015 Ford F350 Lariat FX4 6.7 CC DRW
2015 Eagle Premier 351RSTS