Von, it would really help if you explained what kind of rear end your talking about, but lets assume its an 8 3/4. The pinion was probably not installed correctly with the crush sleeve being torqed to proper specs. This allows the pinion to tighten up changing the alignment with the ring gear (backlash). This is why most rear end rebuilders use a spacer on the pinion and toss the crush sleeve. This is a common problem. There are other reasons the rear end can howl but this is most likely on a new setup.