[69hugger]

The noise isn't limited to 9" Fords. I had a 12 bolt Chev in my last Camaro that had the same issue. I had them replaced with a set of aftermarket gears (don't recall the brand, and it doesn't seem to matter). Had them set up by a local guy that was a GM driveline mechanic for over 40 years. He has done literally thousands of rear ends. They howled like mad. He shrugged his shoulders & said it's the gears. I could buy another set, but I'd likely have the same result. Same story Mark said...they make some good sets, and a bunch of bad ones. I spoke to every gear manufacturers' reps, and they all said they hear noise complaints all the time. This was 15 or so years ago.

I read somewhere many years ago 9" Fords were quieter because of the greater hypoid angle. Seems to make sense that the greater hypoid angle provides the opposite effect of straight cut gears, which are extremely noisy.

My current Camaro has a 9". I had a 60-70 mph neutral-load whine which drove me nuts. I finally decided to have John's (they advertise in the GG Gazette), build me a QUIET pumpkin.
I was told they have a testing vehicle that has microphone(s) mounted on the axle and speakers in the cab, so they can listen for gear noise like with a stethoscope. For a nominal extra fee, they will install your pumpkin, & drive it to make sure it is quiet before shipping. I thought that was no brainer. They also told me they keep track of gear supplier lot numbers, and when they get a noisy set, they log the lot# and black list that lot. This way, they have a pretty good idea of good & bad sets.
Well, my pumpkin was returned to me in late fall, and when I drove it, the gears seemed perfectly quiet. I only put maybe 20-30 mile on them before winter storage. The next spring, aflter driving maybe 50 miles, the howl was back. I called John, and he said this has been happening, he thinks it is related to heat treating. He said he is going crazy trying to get good gears from ANYBODY, but he would stand behind them & give me another rebuild. This time he wanted to put a 3.60 set in instead of the 3.50's I had because he said he had a known good lot. I agreed & he sent me a very quiet (so far) pumpkin. I have several hundred miles on this one, and so far, so good.

Sorry for the long winded reply.

Bill