My 99 has a definite wom-wom-wom sound when the 4 Wheel drive is engaged. And yes, it's when it's in 4 High on the highway, and 4 low when I'm stuck in the snow. Only makes the sound when it's moving btw. Any ideas on the problem? Low fluid or something? 5 Speed manual tranny also, almost forgot to say that!

 

Do you have stock tires or aftermarkets. That is the EXACT sound you would here when riding on 31's or larger

 

Stock tires actually!

 

I get a similare noise in my 97 f150 4.6 4x4 auto trans. I plan on doing a tcase fluid change when it warms up, and front diff fluid. So any info is a great help.