There is a "ringing sound" inside the cabin, it.

There is a "ringing sound" inside the cabin, it seems to be coming from the back seat area, when I drive the truck at higher speed on the highway. It is somewhat intermittent although it was constant the other day. The road surface is a bit rough as the highway is somewhat grated. My tires are old, good tread still but they are the originals and have surface dry rot (will replace them next week). Had rear brake pads replaced. Still the noise. The road is like if you would have loose nails in a glass jar that were vibrating. I'll check for loose parts everywhere and can't find a thing. This started a few months ago. Any ideas?