2000 ford expedition awd eddie b, package. 145k miles. My rear end has hummed for a little while. Today I drained fluid. Refilled 3quarts 75-140 synthetic, 4oz friction modifyer. It is much better after 20mile drive, humming stopped under 40. Hums now 40 to 55. Havent got it over 65. Seems also to have stopped slipping under a load. The noise doesn't really get louder going faster it sometimes has bursts louder then back down. Not like a bearing worse the faster you go. Anyway any recommedations? Should I add 4 more oz friction modifyer? Also heater core is leaking I am very good at repairs done it my whole life just not under dash heater cores is it that bad how long does it take?

 

The humming noise is damage (or wear) already done. The only solution is to rebuild the rear end. For the heater core...the entire dash has to come out to get to it. That takes a good 8 hours if all goes well and you have half a clue when it comes to car work. If you decide to change it as opposed to using stop leak, I suggest you do some research on "broken blend door". With everything torn apart you will want to have new parts for the blend door and replace them.

 

Thanks. For your help.