how did you go welding it?
spatter from welding can get on the crown or pinion, causing a hell of a racket.
but i agree with the others, the heat has distorted shit.
when you are on light throttle, its giving just the right amount of torque to put the pinion at the "sweet spot" and make the noise. my nissan junk daily does the same thing, light throttle on the freeway at 100km/h and there is a whine. go to half throttle and it goes away.