How do you determine if whine coming from front or rear diff?

[Machete]

Beats me but hearing a little more diff noise after 30mph up to 75mph and increases on deceleration.

Can’t tell if coming from front or rear.

Front diff oil fresh. Rear diff oil old. Don’t want to make assumptions.

 

[Fastcar]

Beats me but hearing a little more diff noise after 30mph up to 75mph and increases on deceleration.

Can’t tell if coming from front or rear.

Front diff oil fresh. Rear diff oil old. Don’t want to make assumptions.

Put it 4wd and listen, then 2wd and listen. The difference should be clear

 

[Machete]

I have 3.55 LSD but don’t know size of ring gear.

8.8 or 9.75?

 

[BMW2FORD]

In the auto business we use a tool called a chassis ears. Check this kit below from Amazon. You can clip leads on the front and rear diffs as well as other places and drive the truck switching channels to the various leads and see where the noise is most pronounced. Sometimes driving the truck on a lift and listening while under it works but rarely since most time you need a real load on the drivetrain parts for it to happen which is where this kit helps.

STEELMAN ChassisEAR Electronic Squeak and Rattle Finder Auto Diagnostics Tool, 6 Microphone Clamps with Leads, Over-Ear Headphones https://www.amazon.com/dp/B000IHIAE...abc_SKKWR3B7K169NYH05Q52?_encoding=UTF8&psc=1

 

[Machete]

It’s 9.75. All 5.4’s are 9.75