fawey,

If it's not related to braking or steering, my guess would be suspension. The rubber bushing(where control arms mount to frame) may be dry. Your suspension squats or lists doring speed changes and that may be what you're hearing. My truck gets pretty squeaky in the winter and goes away in the spring. You could try spraying these areas w/ wd40 or something like that.

One more suggestion; put your truck on flat ground, engine off, in neutral. bounce the truck push it, move the steering wheel. See if you can replicate the noise. You may be able t oisolate the problem that way.

If nothing works- TURN THE RADIO UP!!!

good luck, jim- let me know if this helps