This year has been horrible for flat tires. While repairing my latest rear flat today, an explosive pinch flat on a massive pothole ("spring" in Pittsburgh is measured by the potholes and not the weather), I noticed my rear hub bearings were loose, so I investigated.
I examined the drive side cone and it had a pit in the surface. Maybe that's why it was so noisy. I found a spare cone and started replacing it... but then I looked at the other cone and it was way worse: it had pitting all the way around. The hub was still packed with grease (no dirt), and had only 7-10k miles on it. (I didn't pull out the balls and examine them.)
Maybe the damage was caused by the fact that the hub bearings were loose, but I'd expect them to be loose after so much pitting and wear on the bearing surfaces. In any case, I've never heard of bearings loosening while installed on the bike.
This isn't my first problem with this wheel set (it even matches!), either. Within a year after buying these wheels, I had a horrible noise in the front wheel, and investigation revealed that the balls themselves were severely pitted. That wheel hasn't had any problems since I repacked it with new balls.
These are Shimano 105 road hubs bought new only 3 or so years ago. I thought these were supposed to be better than off-brand hubs. Maybe 7-10k miles is "better," in these days when department store bikes are ridden an average of 20 miles total, and "real" bikes have their components upgraded every year to keep up with changing fashion.
37 minutes ago