Horrible grinding noise coming from rear diff

[Ollyh2015]

Just recently purchased a 99 sti full forged running 400hp it's my first scooby so don't know much about them tbh, it's started to make a horrible grinding noise from the rear of the car, it does it when I let the revs off and it stops if I hold the clutch in, after around 40/50mph it doesn't do it at all only at low speed, I used to have a similar noise in my starlet every time I let my foot off the rev that turned out to be the exhaust loose but someone said it sounds like the rear diff just looking for some input! Any is appreciated thanks in advance

[quit]

Sounds like diff is shot , new box needed