Hello, my name is XXXXX XXXXX I will try help you with this.
This is caused by the air flaps inside the manifold.
You can either fit a new manifold or remove the one you have and then modify it by removing the flaps.
The flaps are there to improve low speed torque by lengthening the air tract.
You won't really feel much difference though by removing them. Infact you might get a bit more power but I wont guarantee that fact.