98 1500, really bad whine, rear end/transfer case?

Hi, A friend of mine has a 98 1500 4wd, the truck has a horrible whine to the drivetrain, it's either in the rear end or the transfer case. The faster you go, the louder it gets, easily heard inside, and pretty annoying. I know it's not the fuel pump..I don't think you can even hear the fuel pump over this..lol Anyway, I'm having a hard time telling if it's in the rear or the transfer case, I'm kinda leaning to the transfer case but I'm just not sure. I was wondering if there was any suggestions anyone had. The truck only has 61K on it, all fluids are filled, the rear seems to be pretty tight asside from a little backlash which I know is normal, and I know it's not backlash noise, it much louder than that......Thanks!!

does it make this noise when its in 4WD ?? according to exactly how much backlash you have will determine how loud the gears will be back lash has to be exact or if its too much backlash it will make a noise..an easy way might be to take the rear drive shaft out (you might have to jack the rear of the truck up to keep the tranny fluid in and use jack stands !!!) and put the truck in gear (while running) and just let it shift once or twice then if your noise is still there its the t-case or possible tranny but this will eliminate the rear end... when it shifts out and you see what the noise is shift the transmission to neutral and switch it off...if you dont do this the transmission will grind and break something because there wont be anyway of stopping the shaft inside the tranny except for switching the truck off !!!

Good idea!, thanks, I didn't even think of doing that...

Anytime....just be careful !!! let us know what it is