A week ago, I noticed a knocking sound after about an hour of fairly hard city driving. It sounds like something is loose under the truck. For a couple of days, it seemed to get louder and more frequent, but now seems to be staying the same. I can hear it in the cab, and I can feel it in my feet. It appears to coincide with bumps on the road. It diminishes with light brake pedal pressure and goes away nearly entirely with firm pedal pressure. I have shaken everything under the hood and under the truck. The front-end components, brake components, shocks, exhaust system, sway bar and everything else I could think of/get my hands on is tight. Any ideas?