strange whining sound

[10ga]

All of a sudden my 1998 SR5 4X4 starting making a low pitch sound while in drive but goes away when you put it in park or neutral. It almost sounds like a power steering pump whine but it's not the power steering. You can feel it through the gear shifter and the floor pan. Any ideas what this might be? It just started all of a sudden. The only thing I did right before it started I put it in 4 wheels drive high for just a minute to get up a hill in my yard. Hoping it's nothing serious. Thanks

 

[zumogo]

Check transmission oil, and hope it's not pink.

 

[JayRolla]

Like stated check fluid of radiator and trans before driving anymore. If pink dont drive it and research "pink milkshake".

 

[10ga]

Checked those first thing and transmission fluid is clear and full

 

[Arrowhead]

And after you check all the above, if it is still making sounds, check your flexplate. Just an idea

 

[10ga]

Also, don't know if this has anything to do with it but the sound changes when my AC compressor kicks on. Any chance it would have something to do with the AC or is it just because the compressor is changing the RPM's?

 

[10ga]

Took it by my mechanic and he thinks it's the AC compressor. The sound changes when it kicks up and down and you can feel the vibration in the AC line.