The shoes should just 'touch' the drums .if you manually spin the wheel quite quickly and it continues to rotate a couple of revolutions then its OK, if it grinds to halt within one revolution then they may be set too close , The shoes may not be 'centralised' ( they have quite a bit of freedom to move about ) and before doing the spin test , apply the handbrake two or three times and release then do the test. Has the caravan been recently serviced?.If not you may be hearing a lining thats come off the shoe.