Noise from driveline in 4X4 mode

Hi guys, just bought a Tacoma after having Ford Ranger's for 15 years, so you will have to advise if this is normal. Purchased 2013 Tacoma October 2012, noticed noise right from day one. I have used 4x4 mode most of the winter due to most snow since 1930s, any way I have a drive line noise with slight vibration when driving in 4x4 noise comes in at around 70 kms per hour & seems to be more pronounced around 80 to 90 kms ph, I read the manual & it says not to drive over 100 kms per hour or 60 miles an hour, the noise seems to be getting worse & I have taken out of 4x4H going along and the noise goes away. Booked in with the local Toyota dealer at 6,000 kms got them to check it out, they tell me it is normal & the truck is under warranty & keep driving it, they also suggested if I wanted to go further then I should contact Toyota direct. can some one shed some light on this noise or is it in my mind? any feed back would be great, I have 8,000 kms or 5,000 miles on the truck now.