Have noise in 2006 Z71 1/2 ton crew cab sierra 1500

[enjoy425]

I have a noise in the front end and I have had it to a transmission place ,front end place and no one seems to know what the noise is or were it is coming from,the noise starts when I hit 80km (50mph) and it never gets any loudier the faster you go it seems to stay the same,it sounds like a rubbering noise or a oooww sound ,the mechanic says it is the stranges noise hear ever heard ,he put it up on a hoist and check everything possible and found nothing he thought it may be wind noise but they was nothing loose underneath,I have brand new tires so it's not tires and new brakes ,wheel bearings,and if you go below 80 kms the noise is go anyone have any ideas ,not knowing drive me crazy ,the mechanics don't feel there is any danger in driving it because the front end and tranny and axles are all tight ,I have 98.000ks on it with a 5,3 litre engine ,if you can help it would be appreciated
thnaks