Vibration and noise PLEASE

[72chevyLWB]

I am totally confused here. I went on a road trip with my 72. Out on the highway at about 65 it makes a whah whah whah kind of noise (dont laugh) and a vibration comes with the sound. Does it sometimes but not all of the time. If I am really steady with the throttle it won't do it, but if I move the throttle around any it will. I have a 3.73 rear end. I have a 3 series carrier with a ring gear spacer. I checked the pattern on the gears and everything looks good. It has a 1 piece driveshaft that is straight and all u-joints are good. It did that for a while, but all of a sudden one night I have this nasty vibration that shakes everything inside the cab. Anything over 45mph it vibrates no matter what you do, even slowing down. It is also really bad when put in neutral while slowing down (almost gets worse). My wheels are balanced and there are no thrown weights ( I checked). While I had the cover off I re-torqued the ring gear bolts because you have to with the spacer and I got 1/8 to 1/4 turn max out of it so that shouldn't be the problem. I have the original open carrier and might have to throw that in and go from there. Any other help or ideas would be appreciated.