8.75 Axle vent

The vent cap on top should wobble around and you should be able to blow through the passageway fairly easily. Use some carb cleaner and run a small wire up inside to see if it will loosen up any crud inside then hit it with some compressed air to blow it out. If you have to, pry the cap off and clean it out that way. Just remember to crimp it back on so it doesn't fall off in the road.