I recently bought a 21 speed city/urban bike and want the freewheel to move without sound. In the past on older mountain bikes I found putting oil in both sides of the rear axle worked. But I've tried it this time without success. My 6 year old hybrid has a silent freewheel but I can't remember if that was oiled or it was like that to start with. Are some/modern freewheels sealed at the hub or something?