the past week ive been working on my car installing dual calipers and a hydro e brake. And to do so you have to open the diff and pull out the axle shafts. I refilled the diff with 75w90 instead of 75w140 because I accidentally bought it after some poor research and decided to fill the diff up with it anyways. Could this be the issue? I also have a video of the sounds if anyone would like that. I very concerned and scared lol
some of the sounds I heard was gear winding loud rotating sounds while driving in a straight line etc.....
I donīt think 75w90 would ruin a diff right?

 

and yes I did use 4oz of limited slip additive

 

its a 2013 mustang sorry im new to forums lol

 

using the wrong weight of oil can have unforeseen consequences like premature wear (failure), noise and other issues.

There is a suggested weight (viscosity) of oil for a reason and that reason is that it meets the required performance specifications. 75-90 is too thin. The noise you're getting is most likely due to metal-to-metal contact of parts that normally have a layer of oil between them. At high temps and high revolutions (like in a diff) the 75-90 is just getting flung off all the rotating components and not sticking.

The other possibility for whine (though I don't see this being the cause in your case) is gear whine from incorrect pinion angle of the driveshaft.

This usually occurs after you lower the vehicle or do drivetrain work and replaced things like U-joint, driveshafts or rear end suspension. .

The only way to 100% say that the oil IS the problem is to swap it out with the right weight and confirm the noise goes away (unless you've done drivetrain work as mentioned above).