Rear end noise, can't figure it out!

[AirSpeed]

I have an 87 K5 I don't drive often. I drove it today and heard a noise coming from the back of the blazer, I thought it was the parking brake rubbing so I took off both dorms, cleaned the brakes and backed off the parking brake adjuster, noise still there. Next I jacked the rear end and spun the tires manually and can clearly hear a soft dull thud sound that increases with speed. I had a friend push the brakes to see if the sound would change, it didn't. I checked the u joints and they are good. I feel no vibrations, only sound. Any ideas what I should look for next? Any help diagnosing this would be greatly appreciated!
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