just bought a 47 chevy streetrod, great car, but at speeds over 30mph on acceleration and keeping aconstant speed and under accerlation a howl becomes obvious, if acceleration is let up it disappears? what is going on? the rearend is out of a 79 camero, 10 bolt posi, supposedely rebuilt? sounds almost like a vacuum leak but all lines are OK?

I cant hear the noise but the cat was let out of the bag when you said the rear was suposedly rebuilt . I know for a fact 9 out of ten people rebuilding rears have no business in them . The noise you describe comes from improper backlash and gear mesh mis alignment . 95% of the time people swap- gears and dont put them back exactly as they came out of the doner rear . In other words when you are accelerating or holding a constant speed the rear Howels due to the gears running in a new wear pattern that normally causes extream vibration , the vibration causes the Howel , its rare you can actually feel the vibration but I have seen some so bad that it was felt in the drivers seat . When you let off the Load is removed from the gears and the niose goes away . I have found posi rears will amplify gear noise due to the extra drag of the limited slip unit . If it has a low gear it will be noiser as a rule . Setting a rear up takes time and patience and most of all , the proper tools to set the backlash and preload on the pinion bearings . Any time you remove bearings or ring and pinion sets you must take special care to keep the bearing races and bearing spacers with the side of the carrier they came from , If you replace a bearing you must replace the race or it will make some noise . never cross breed ring and pinions ,, keep sets together , in other words if you pop a tooth on your ring gear you have to replace the ring and pinion , not just the ring . My advice is to ask around in your area for someone that has a good reputation setting up gears . Let them pull the cover and check side to side clearance and back lash . I wouldn't be suprised if you have to replace the ring and pinion set .

This sounds to me that you have a out of alignment of the driveshaft to the rear from the trany check with a small level put it on top of the drive shaft to see if it is level if it is out it will ware out the universal joints and it will cause a wine noise when you let off at exceleration it will pull down making it more leval and you wont notice it as much but you will ware out your front universal joint firstcheck the play in the front joint also if the drive shaft is out of leval shim out the tail peice on the trany.