Noise form drive train gets quieter when in Auto 4WD
 

When traveling at highway speeds I can here a noise coming from the drive train. Its not real loud but it gets less noticable as soon as I push the Auto 4WD button and as long as I leave it in the noise stays at the same level but as soon as I put it back in 2WD it get louder.....I'm lost.

Truck info=2005 GMC Sierra 1500 CCSB SLT Z71 stock everything except rims and tires 55k miles.

Being that it's not real loud I wouldn't worry too much until it does start getting louder. By going to Auto 4WD you are engaging the transfer case and being in Auto 4WD I don't think the front differential is engaged until there is slippage at the wheels. My local service manager does not recommend driving on the hwy in Auto 4WD because he says it can wear out the clutches in the transfer case.

Thanks, its good to know about driving in auto 4wd. I would like to find out whats causing the noise before something goes and leaves me stranded.