I am not an expert, and don't claim to be...BUT, I have had personal experience with this exact same issue. Although it may look good, the pinion bearing is probably bad. The rear in my car was doing the same thing, I replaced everything in it except the pinion bearing. Everyone told me that the pinion bearing was OK. They replaced every bearing in it, BUT the pinion bearing, don't ask me why, I dunno. They told me it looked OK, but it howled just like you described until it was replaced.