I've got a whirring sound in the right rear wheel. It's been there since I bought it a year and a half ago, but I'm curious what it could be and how dire it is to be fixed. Also I looked down there today and there is a buildup of old grease on the inside of the drum back there, that isn't on the left hand side. Wheel bearing? Other? please help, thanks!

 

Possibly rear axle seals and bearings.

The bearing starts having excessive play, which makes the axle wobble some. Which then eats the seal.

 

Is it common for it to be only one side like that? the sound is definitely only coming from the right rear. When I slow down and turn down the music, it isn't constant. at speeds below 20 it almost sounds like something rubbing on the wheel.

 

Right rear is loaded slightly more because it is on the low side of the road.

More importantly, that wheel is your "pushing" wheel when accelerating. Therefore it has more loading than the left side.