My truck makes a howling noise only when I'm accelerating from 40 to 45 mph up to about 60 mph. The noise sounds like its coming from the front of the vehicle. Is very consistent regardless of engine temperature or outside temperature.
You're going to want to check your wheel bearings. Wheel bearings go out on these truck after a while ... I'd assume your '04 Avalanche has a few miles on it. I've changed them out on my '06 Silverado Z71 which has the same suspension on it. If you replace one, consider changing both of them. I wouldn't drive it too awful much until you get it diagnosed, though. Just jack your front end up and check the wheel for play. Top and bottom, then side to side. It should be pretty evident.