01 ford explorer sport trac lf wheel noise

While in 4wd and on the gas, everything seems normal. when I let of the gas and coast, the lf wheel/tire make a loud noise--seems to be a rotating/grinding or whirling noise. I suspect the wheel hub/bearing may be the culprit, but upon inspection, it seems totally fine. It does not matter if I am turning or going straight. Any suggestions?

I got the same explorer sport, except mine isnt a trac, I've also noticed the car is louder when it's on 4x4, its probably normal. I usually drive it on rear wheel though, it saves gas..