Close the vent, if necessary jam something underneath to stop the noise coming through.

My dd went through a phase around the same age when she was convinced the bed was moving & there were strange lights on the ceiling. We worked out the movement inside the colour-changing nightlight was creating moving shadows. Solution, change night light.

Is it only when LO is asleep? It might be the sensation of being woken from light sleep that is scary, rather than the noise itself. You could try opening the vent again in a couple of months, it might not be an issue by then.