I find with rabbits aggression (or grunting and lunging) is usually from one of 3 things….
1. Stressed/Scared (rabbits usually act out of fear… they learn that lunging and grunting gets the scary kids to go away, so they keep doing it)
2. They are hormonal and being territorial
3. They are sick.
Its’ hard to say what it is until you have him as it can also be a combination of them.
Good for you for taking him in and preparing yourself ahead of time
I would suggest a vet appointment as soon as you get him to double check if he’s a boy or a girl and if he is ok health wise and possibly book his neuter.
If its mostly hormonal and territorial than getting him fixed will help with most of that.
If he’s just scared and nervous than just letting him get settled and leaving him alone for a bit will help a lot. Get him settled and then don’t try to interact for a week – just feed him, give him the odd treat or his nightly veggies, change his water and litter and just talk in a calm voice.
Once he’s settled just sit on the floor and ignore him and let him come to you.
I found with my boy it was a combination of hormones – which went completely away when he was neutered – and being scared.