Yes, air flow lose when under acceleration. I think I've must've put too much pressure on acceleration while the AC is on.
Nah, that means the 2-port vacuum motor has perished, it's not caused by any excess pressure on acceleration.
Your engine provides vacuum to the system on overrun. When you accelerate, there's less vacuum for the system. A leak in the system (from a perished vacuum motor) quickly drains the vacuum tank, so the shutters change position. The hiss you're hearing is from the perished seal.
See that vacuum motor (in the pic below) with the white 'X' on it?
That's the source of your pain.
As you can see, I've had the ordeal myself of replacing one.
Pic below was taken mid-ordeal, since replacing / repairing it is a dash out job.
(Well, this was in a 940T, but that part is all the same).
The Beever, which @VolDan now owns, has this same problem.
As does The Diva (965), which I've only just discovered, after getting the aircon working again.
You'll either need a new 2-port vacuum motor, or a seal which @asiandriver had custom manufactured and now sells, to fix it.