High Speed Audible Rattle

[hawkeyeflyboy]

I drive an '04 Z71 Silverado Crew Cab with 73K miles. I've been hearing a rattle seemingly coming from the passenger-side undercarriage near the bell housing. It only happens at 45mph or faster. Conventional wisdom points to some sort of shield rattling due to wind, but I've put it on the lift and checked/tightened all sheild mounts and nothing seems out of the ordinary, yet it still makes noise. It happens every time I drive and is quite loud. There is no pattern to the noise, just a random rattle sequence with no consistency. I'm fairly certain it is due to wind (does not seem to be related to tranny/engine) at high speeds but I'm at my wits end on where it is coming from. Any help would be appreciated.