There are two bearings per wheel, just like modern cars, but they are ball instead of roller. Try adjusting the bearings first. It's kind of like adjusting the bearings on a bicycle. That may be all you need. If the wheel makes noise after you adjust it, then you will need to replace one or both bearings on that side. You should clean and lube the bearings too (both wheels), if they haven't had any attention for a while.
Ray
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