I drive a 2002 GMC Sierra. When I first got it, I could hear a odd metal sound coming from the "backseat." It was still under warranty, so I took it into the dealership. They ended up lubricating something (I think brake related), but I'm not sure. After that the sound was gone. Only recently I've begun to hear it again. So, my question is it the chassis that needs to be lubricated or another part of the braking system?