1996 tahoe noice from front diferential

I have a 96 tahoe the unit is in excellent condition. I bought the unit used 2 years ago with 150,000km. I just began to do a little 4x4. Funny thing is when I put the unit into four wheel drive I get a noise coming from the four wheel drive system. I just took it in the shop and they told me that the noise is normal. The mechanic told me that the chevy's were proned to this kind of noise and that when the trucks first came out GM was changing out the front diff's only to find out later that the noise did not go away. They then tried synthetic oil which didnt solve the problem. He basically told me its normal. Anyone out there have the same problem and if they have had any resolution to this.. looking for answers:

Yea I an excellent 96 yukon with 76,000m and I just recently had the front seromotor replaced and ever since that right after i take it out of 4wd the front diff makes a grinding sound. But after it sits like over night it quits. And when it is makin the sound the 4wd wont go in. Any suggestions?