Differential whine? I just bought a 2006 Chevy Avalanche Z71. It has a whining noise when cruising 40-75 mph. I thought it was mud tire tread noise but still there after getting standard equipment tires. What is the most likely cause

Could be your alternator or a bad idler pulley. Is the transmission good? It may be the tranny or your differential whining.

check to see if there's anything loose underneath the car that could get pushed into your tires as you speed up it happened to me it was the wheel well plastic guarding that was torn off and hitting the tire at faster speeds.

have you looked at the tires- if they are wore down the wear bars-little bands of rubber that cross the grooves in tread will be exposed and the noise is your warning to get new tires soon

take off belt and try again onely for less than 5 min to see if it still makes noise

Could be tire noise, could be rear wheel bearing, differential (if rear wheel driven), or could be something associated with the brake.

Try swapping the tire with another tire. If the noise moves with the tire, it's the tire. If it stays, it's brakes (unlikely for the noise you mention), rear wheel bearing, or rear differential (lack of oil). Will probably need to have a mechanic to identify and repair.

could be power steering pump low on fluid

i'm thinking that your tires need speed balancing :),, if is not the tires then one of your bearings on the hub needs replacing or the exhaust has a hole somewhere.

Sounds Like Pinion Bearings , But Carrier Bearings Should Be Replaced (In My Opinion) as they Could Be Contaminated

Check the condition of front disc brake pads, if they are in need of replacement a preset little metal arm starts rubbing the discs and will make a whining noise. It doesn't do any damage but it's irritating to listen to.