Metal/Moaning sound at 30-32 mph only

[700]

First time on this great site.

Metal/moaning sound at 30-32 mph only. I have a 2015 Gmc 2500HD with 73000km's / 45620 miles with 17" duratracs - (at 30-32 miles per hour only) in 2wd /4wd, in neutral or in gear & speeding up or slowing down I hear this slight metal moan coming from the front driver’s side and only at this speed. Tires have less than 7500 miles on them and the noise only started about 3500 miles ago even tried rotating them. brought it to a GM shop and the shop foreman and I went on a test drive and he said he never heard this before and had no clue what it is.

I have put the truck up on a hoist at least 3 times in the last 6 months and it doesn't make a noise when we try it on the hoist in 2wd or 4wd.front end and bearings are all good. all  fluids have been replaced in the diff, transfer case and tranny 

 

Any help would be greatly appreciated.