Transfer Case Or Front Diff Noise

[04sierra2500hd]

i have been noticing that my truck has been making a wining noise lately. i this its either the front diff of the transfer case. i have an 04 GMC sierra 2500HD with the 6.0l and an automatic. it has 29k miles on it. the noise goes away when i put in 4 wheel drive(has the push button transfer case). the weird thing thats driving me nuts is that it is not a constant noise. its an on off noise (wining noise).and it doesnt speed up when you inscrease the speed, it is very constistant. i would think that if it was the front diff or the transfer case the noise would speed up/happen more frequently as the speed increases. but it doesnt. this morning was really bad. it was abou 12*F to 14*F outside. it was making the noise pretty loud(i could feel a vibration in the gas pedal, and thats never happened before). i put in 4 wheel drive and the noise went away. put it back in 2 wheel drive and the noise came back. it lessend about 8-10miles into my trip to work this morning. i checked the fluids and they are right at level. i just changed all the driveline fluids at about 25-26,000 miles. i switched everything over to synthetic because i tow with it in the summers, plus i heard its better for the truck in the cold weather. has anyone else had this happen to them. is this a common problem. any info would be a great help. thanks alot

 

dan