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Limited slip differential noise

My Chevy 96 Impala SS makes a loud noise that comes from the limited slip differential. The noise started after I changed the oil in the rear end. I have used Lucas 80W-90 oils and about 5oz of an additive for limited slip differential. After driving the car for 10 miles the differential started to make noise. I thought, perhaps I should not use the additive with the Lucas gear oils or perhaps I added too much of it. I have removed about 1 quart of the oil and replaced it with 1 quart of plain oil, the noise did not go away. Next I have opened the differential and remove all oil and filed it with plain 80W90 oil no additive for limited slip was added. The noise is still there, it is especially loud at very low speed while making turns (speed as slow as very slow walk). Before I change the oil everything was OK.

Any suggestions what to do next?

Are you sure you used the right weight oil? Did it need Hypoid/non-hypoid and did you use the other one?

80w-90 is the correct weight oil. What additive did you use? The specs call for 4 ounces friction modifier. I would be using oem modifier to keep the confusion down.

I used 80W-90 GL5 oil and an additive for limited slip differential. There was only one additive to choose from on the self in the store, it comes in a tube and looks like a big tooth paste, but it is a liquid. I am not sure what brand it was.