rumble/roar, louder under deceleration, loudest under braking

Accelerate or cruise and you can't hear anything. Coast in gear between 20-60mph and you hear it a bit, pitch changes with speed. Brake and you can hear it over the radio. There's a little vibration through the floor, but not through the steering wheel or brake pedal. I don't think it's brake-related.

Some background on my truck: it has 42,000 miles, but it sat for maybe three years unused, sitting outside in upstate NY. The frame was replaced but the front suspension and rear axle housing are incredibly rusty.

My instinct is that a noise like this is usually driveline-related. I checked the pinion bearings when I changed the pinion seal, they're tight with the correct preload. Changed the diff oil, no change in sound, though the old diff oil was gross (no significant metal shavings on the plug, but it was gray and foamy - contaminated by water?). Driveshaft seems fine, I have a regular cab with no center bearing. There's no play in the u-joints.

It made this noise before I touched anything but nothing I've done to the diff has changed it at all. My only other thought is wheel bearings, though it doesn't change when turning left/right so it's probably not the fronts.

Due to finding a crack in the diff housing, I'm replacing the whole assembly with a much cleaner, not-rusted-out unit, so I'll see if that fixes it... but if anyone has heard this before, I'd like to know what they did to fix it.

 

Figured this out. I have a tight rear u-joint since the truck had been sitting for a while.

Hitting it with some penetrating oil to break up the rust on the outside of the joint seems to have loosened it.