I have a fairly loud hum once I hit 110 km/hr (65 mph). When I let off the accelerator it stops altogether. Then I hit it again to keep the truck at 110 and it will start again. Any idea what could cause this? Thanks.

Maybe u-joints? Just a thought for starters, I see you have an 07 though.

Check your tire wear on the inside of the front tires. Mine was doing this also and had tires rotated and front end alignment and noise went away. I only have 17K on the odometer, but it fixed the humm

All good advice... something else to check is proper tire balancing.

A place to look is the rear axle. I don't know if it would be what your problem is, but my dad had a similar problem on his ford powerstroke. We looked everywhere to find the noise. He had to have his axle rebuilt, I think it was just new bearings and seals.

check the U-joints first, tire balance and don't forget to check the front driveshafts. it a new hum or has something been changed?

Sorry, was out of town, yeah it's a new hum. I just drove it out to BC and back, about 1200 km with no problems. The U-joints were good and as for the tires, the truck drives straight with no shimmy at all. Tire wear is normal. I'll have the local shop take a look and I'll let you know what they say. It's at 48000 km and I have warrantee to 60. Thanks for the help.

Sorry, was out of town, yeah it's a new hum. I just drove it out to BC and back, about 1200 km with no problems. The U-joints were good and as for the tires, the truck drives straight with no shimmy at all. Tire wear is normal. I'll have the local shop take a look and I'll let you know what they say. It's at 48000 km and I have warrantee to 60. Thanks for the help.

Exhaust drone (many 06's were noted for it at 65mph especially cc/sb)
Tires
U-joints
Rear-end

Try engaging 4wd and see if it changes at all.

If any above don't work, it may be the rear end's back-lash. I know its knew, but you never know. GM has been known to forget to check things, like how much lube (if any at all) they put into the pumpkins....

I put off getting it into the shop last month and towed my fifth wheel last weekend. The noise was alot worse when towing. Almost like driving over rumble strips. So I get home and the next morning noticed tranny fluid in my garage floor. Took it to the shop this morning and they replaced the yoke on the transmission. I'll pick it later today, hopefully that fixes the highway noise. Does anyone know what would cause this seal to go? the truck only has 50,000 km on it.