Re: 2008 Expedition Acceleration Noise
If the vehicle isn't showing any codes it'll be hard to diagnose of course. Still, I'd get them to check out the driveline (axles, transfer case if 4x4). Could be a slipping U joint that you're hearing. With it being a new vehicle it's unlikely. If you have more than 30K it's advisable to do a transmission fluid and filter change as well. I'm sure the dealer may drop the pan anyway to see if the vacuum sound you hear is related to the valve body. I'm not a tech by any means, just a gear-head giving my two cents.
Oh I nearly forgot: If the dealer has a similarly equipped Expedition on the lot, ask them to take it for a test drive to see if you can duplicate the problem you have with your vehicle. If so, then I'd contact Ford directly about the problem as the dealer may be useless in helping you out. They'll just tell you it's "normal" and send you on your way. Fight it til the end.
'00 Durango R/T 360ci 290hp (modded); 138,500m
'06 Pontiac G6 GT 3.5L 220hp; 44,000m
'12 Chrysler 200 Limited 3.6L 283hp; 13,000m
'99 Taurus 3.0L 2V Vulcan 145hp; 154,300m - Traded
Amsoil in all vehicles!