8-3/4 Rear High Speed Hum

Tech Question


neil schroll, Mount Joy, PA, 1969 Dodge Charger 440

I have a high pitched hummm/whirr eminating from my 8&3/4, 742 casedifferential at about 100 mph +. When you drop back below this speed, itgoes away. How do I fix this problem with out always driving slower than 100mph???

The rear was recently set-up & rebuilt with all new bearings, new clutchesfor the posi, new axle bearings, and a good used set of 3.23 gears. I alsohave a new drive shaft from Denny’s drive shafts. The transmission is the727.

I appreciate any help/advice you can give me on this matter.

Oh, and by the way, great magazine! I received my first subscription thispast December, and I love it. Keep up the great tech work.




There’s really ony two places that rear axle noise can come from: thebearings, and the gears. Typically, bearing noise is a whine or growl thatincreases proportionally in frequency and volume with road speed. Gearwhine, however, can vary with road speed in an almost random way, and canoccur under drive, coast, or both conditions.

Setting up used gears is a touchy situation. Once they are broken it issometimes impossible to re-install them and have them be silent — andsometimes not! I suggest dropping out the carrier and having a good, closelook at the gear contact pattern. Try re-setting it to get it as close tothe book — or the original wear pattern — as possible.

If this doesn’t help, as long as the gear contact area isn’t bottoming out,or running off the edge of a tooth, the life expectancy should still bedecent. (Check the temperature of the carrier, and the lube, after a top-endrun, just to be sure all is well. I’d be unhappy with anything over about220 deg. Farenheit). If OK…you may just need louder mufflers and/or aradio! (Or, of course, another gearset).


Comments are closed.

This website uses cookies to improve your experience. We'll assume you're ok with this, but you can opt-out if you wish. Accept Read More