Vehicle is a 2000 S10 Ext cab 4.3l 4x4 64,500 miles.

Was going down the highway yesterday at about 55 mph when all of a sudden there was this real loud noise. First thought it was a siren but realized not the same type noise. Thought it was vehicle passing but it did not go away after vehicle passed. Exiting the highway the sound would gradually go away while reducing speed. Stopped and looked underneath and saw nothing unusual. When we started home did not hear the noise again until almost home (a three mile drive mixed city/rural). Son-in-law was driving and stated that noise would stop anytime he pressed the accelerator. As soon as he let up on the accelerator it stopped. Anytime the noise started it was very loud. Never felt any vibration is feet while it was making noise.

Any suggestions on what to start checking?

 

Does it make this noise while in park if you rev the engine? Or does it only occur while the vehicle is moving?

 

Two things:
1. Correction to original post. Son-in-law stated sound would stop when he pressed accelerator and start again when he left his foot off accelerator.
2. Checked both front differential and transfer case fluid and both were very low. Bought the truck last Nov and was going to change the fluids this summer but decided to wait until cooler weather in fall due to only 64k miles. I have only put about 500 miles on it since November. Changed the differential today and going to change transfer case in morning.

 

In additon to the items mentioned, check the often overlooked viscous fan. They can make strange noises when they fail.

 

Jimspahr,
Would the noise from the fan be intermittent? When it happened the other day, after we had shut the engine off to go into store it did not happen again until we had driven a couple miles.