I've taken out bearings and drilled a shallow hole where the bearing was worn most. Then with a punch I've knocked the bottom of the shallow hole until the clearance is taken up. With modern oils, this fix could last as long as the original bearing did. Alternatively, just rotating the bearing 180 degrees is sometimes enough to quiet a noisy fan. Another alternative is to replace the bearing and in some cases an Oilite bearing can be sourced, say, from McMaster Carr that will turn your fan into a maintenance free fan.