Whining sound when accelerator pushed, goes away when put into 4wdHi

[Phathardy]

I recently bought a 98 XLT with 4x4, and it whines pretty loudly, being new to Fords and 4x4's I had really no clue what it was. I noticed it didn't really do the whining when in first gear (auto trans) but after first it whined, and it gets louder as i speed up. But when I let off the accelerator it doesn't whine, also when I switch it into 4wdhi It goes away. It's weird cause I can drive in 4wdhi with no whine, then i switch into auto and the whine comes back. Any help would be appreciated. Thanks.

 

[Phathardy]

please help me. The sound is annoying, and I want to solve the problem. Thanks.

 

[Mbrooks420]

If you floor it does it go away? If so, i'd say it is the Idle air control valve (IAC). Dogfriend posted a sound clip of this, if you want to search for it.