I have a 2001 Blazer 4 door 2wd. It has started making kind of a whirring noise at 50 mph and it stops completely at 54 mph. I keep it lined up, and I keep the tires balanced. If I let off of the gas when it is whirring, it stops. No hard shifting in transmission. It is not loud, and it does not sound like metal grinding. Almost sounds like wind blowing through something. Also, I do have a vacum leak. When going up steep hills, air gushes out of the panel vents and stops as soon as I hit level road again (not speed related, only load related).

 

If the noise stops when you let off the gas, it could very well be worn ring & pinion gears in the differential. They can make a howling noise while maintaining a constant speed, decelerating, or accelerating, just depends which adjustment is incorrect. If that's the problem, once it starts making the noise, the damage is done and it will need a new ring & pinion set.

 

It has stopped making the noise and is running fine. I still have the get that vacuum leak checked out. Kind of strange that the noise has stopped.

 

Better hang on to that vehicle. The ones that fix themselves are really rare