Rear end problem help

Even if it's not howling all the time? Only when I let off gas around 40 I hear it

Yes. You've been told several times. We cannot help that it's not the answer you want to hear. SOMETHING has caused the pinion gear depth to change......that is unless it's been making the noise the whole time, then it was set up incorrectly. Either way the pinion gear depth is not right. What it's due TO you will not know unless you start investigating.