I answered the question originally the way I did because you were concerned with bearings, but I've given it more thought. Another common thing that makes noise only in hi range is the direct drive clutch teeth. At least this is the case with a 2J head like mine. I've never had a step model apart. If you put it in hi and grab a hold of the end of the spindle where the R8 taper is, can you turn it back and forth and hear some slop/clicking? If so, this is probably what is making the noise (or at least one thing that is making noise). The step pulley models are a lot quieter than the vari speed models.
Another way to check is to disengage the hi/low lever and try to move it up higher yet and see if this changes the noise. Don't lower it, because you can grind the teeth by doing so if you move it too far; move it up only. Some plates that have the hi range hole in them can be either moved up by loosening the set screw in the side or you can flip the piece over and extend it's range (if it hasn't been flipped over previously.
I would definitely suggest checking this out before doing a rebuild.
Ted