98 gmc sierra ext cab 4wd

I recently lowered my 98 sierra to 3" front & 4" back. the front is lower than the front. I lowered it with a lowering kit that it was special made
for 4wd. But after a couple days i started to notice a sound coming out of the front, only when the 4wd is on. i can only hear it when am going like around 30 to 40 mph, and when am not pressing the gas. I think is getting louder but am not sure. If any one know what it might be please help me...
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