Hello,
My wife's truck is an '01 Expedition with the 4.6. At all speeds, the truck emits a groan/howl that lasts about 1-3 seconds.Does not happen all the time, but the wife is embarrased by it. Wife says it sounds like a ship's horn. Removed the drive belt. It is in good shape. The alternator bearing is slightly noisy.Thinking that may be the culprit.btw,46,000 miles.
Thanks in advance.
galaxieman

Found it!
Dealer replaced the air valve assembly.
All better.
galaxieman