BMW X5 Suspension ?

Just recently I’ve noticed a very loud hissing coming from the back passenger side tyre area of our X5. The left side seems a lot lower than the right and when driving over bumps it makes a really hard bashing noise. But other times it’s fine. It’s my bfs car so I can’t take it to a garage myself but does anyone know what the problem could be? I’m guessing it’s the suspension but he says the hissing is normal and that apparently I’ve pressed a tailgate button?!