I have a Surly Pugsley that I ride for commuting and some trails. Fairly frequently, the bike starts to "squeak" - such as if I'm standing with the bike, rolling into the freight elevator in my work building, and I rock the bike back and forth a few inches - SQUEAK! A now-departed LBS showed me how to fix this with some Tri-flow applied to a part on the rear wheel. I just gently pry back the rubber "gasket" and squirt in some lube. It's a bit tricky to get in there without having the bike on a stand. What is this part? Is there something that needs to be replaced? Please see the arrow on the photo.