Post on behalf of a brother,
I recently bought an Evo 7 with a lot of aftermarket goodies and was tuned by the previous owner. It has an estimate of 400-450bhp, but the drivetrain is stock. Changed all fluids and lubricants after buying it, but the rear differential used to make a very light whining noise. Recently, I changed the differential oil and pushed the car a lot with rough acceleration and shifting, and the noise got a lot worse. I don't want to drive the car like this so I need a suggestion.
Can a standard Evo 7's rear differential handle around 400-450bhp? If so, then will it be ideal to swap it with a new Evo 7 differential? The problem is my country doesn't have a good enough workshop or mechanic to properly do a differential rebuild with aftermarket parts.
And if anyone can help me figure out what exactly is causing this noise it would be very helpful. Here's an audio from the cabin where you can hear the whine after I lift my foot off the gas.

 

Terrible audio clip I'm afraid and your post needs much more information for anyone to even begin to guess what might be wrong.
What exact model, does it have AYC?
Are fluids all good? Any leaks?
In order for the internet to help without just being jerks you must add any and all relevant information.