hey guys. I haven't posted in a while. 4 kids later and a couple moves around the world things have finally settled down a bit.

I'm getting this very weird loud almost spooling like sound (whining, spinning sound) when i'm decelerating or taking my foot of the gas. it happens in all gears and at all speeds. it's almost like when my engine goes into a "deep" vacuum it starts making that spooling/spinning sound. I've checked all the connections, nothing is loose, no CEL, nothing obvious under the hood or under the truck. Specs of the truck in my sig

it happened when i was driving down a mountain. i was coasting down the mountain and in vacuum for a couple seconds, then all of a sudden i started to hear a loud spooling/spinning sound coming from the engine compartment. this has never happened before, and i've never experienced it in any of my vehicles.

any help would be great.i'll try to paste a link to a video i captured when it happens. you can watch the RPM meter drop, then all of a sudden you hear the sound. If i give it a little gas, it goes away immediately, pause a second and it comes back on decelerate.you can hear it from seconds 7-11 then 13- 16, and again from 22-24.

here's the link:

 

Almost sounds like a fan rubbing the shroud, do you have mechanical fans or electric?

 

 

thanks for the replies! i thought it was the bypass actuator for the supercharger. I ordered a new one from Magnuson, but i installed it and it sound it still there. so $220 dollars later, the problem still exists.

however, i did notice by chance when i put the truck in 4WD that the sound goes away completely. just to double check i coasted down a hill in 4WD no sound, put in 4wd and the sound immediately came back. so i'm guessing Transfer Case... maybe front Diff?

 

I listened over and over again on my TV with an excellent sound system. To me it sounds like pulling a long wire rope across the edge of a steel table.

 

how is that info going to help the OP?

 

I guess it won't be helpful unless he has access to professional audio recording equipment. Because of the way phones record audio, almost all of these "listen to this" videos have sound that seems to emphasize everything except what the people who post them want us to hear. I thought my description might help others realize that it is a sound in their truck that they have heard before, too.