4x4 howling

A few days ago it snowed here in Edmonton so I thought it was the perfect time to try out my 4 wheel drive. I put it in 4HI and was on my way but when i started going over 40 km/h a howling noise started coming out of my front differential and the faster i would go the louder it would get. I took it to the dealer because i thought that the differential oil needed to be changed so he did that and he also took it apart to see if anything was broken but said there was nothing. It still howls tho and i dunno wuts wrong. Did anyone else ever experiece this?? Any suggestions on what might be wrong or is this common on Silverado 4x4s?

GM has been having problems with the front diff's and the transfer case. There is a tsb concerning this. Something to do with the bearings in the diff, and/or the clutches in the transfer case.

GM has been having problems with the front diff's and the transfer case. There is a tsb concerning this. Something to do with the bearings in the diff, and/or the clutches in the transfer case.
So what should I do about it?? Is it OK to drive in 4hi or should i just stick to 2hi?

Mine does it too, posted here about it way back and evertone said it is normal. My '98 Z71 did'nt do it, nor did my '04 duramax, but this '99 Z71 does, 75k miles, everthing works fine, just some noise in 4x4--even switched to synthetic lubes all around!
Z400, how you like that magnaflow? been lookin real hard at that system for my truck.

I like my magnaflow a lot bcz its not loud and annoying like some other brands. At idle its nice and quiet so at a red light i can still shake all the cars around me with my bass and when u get on the gas it has a nice "big truck" growl!

Mine does it too, posted here about it way back and evertone said it is normal. My '98 Z71 did'nt do it, nor did my '04 duramax, but this '99 Z71 does, 75k miles, everthing works fine, just some noise in 4x4--even switched to synthetic lubes all around!

So can i drive around with it or what?

You be fine, if it ain't broke don't fix it.

if the diff's howling then the gears must be noisy and that not acceptable. If its under warranty get it fixed