How to sound insulate these air con vents?

Coolaholigah

I have 2 in floor air conditioning vents which for various reasons are no longer connected to the air con system. I have recently had a motorised ventilation system installed in the store room where the other end of the air con ducting is. As that end of the ducting is open to the room I am getting noise from the motor coming back through the ducting and into the room where the vents are  
can you suggest how to insulate from this noise? I could fill the void with rags I suppose but is there some sort of batting which would be better? And then maybe something flat and black to put directly under the frame part that is visible in the floor.

Vent opening is 350 x 150mm at top and about 450mm deep

EricL

Hello @Coolaholigah 

I propose using Bastion 1200 x 600 x 50mm XPS Multi Use Foam Board. I suggest cutting the foam board to the size of the opening and using it as a front cove/sealer to prevent sound from coming out of the vent. I recommend using two covers one on top of the other for a full seal. I then suggest placing a piece of black paper above it so that when the cover is placed back you will not see the board. 

Let me call on our experienced members @Dave-1 and @Nailbag for their suggestions.

If you need further assistance, please let us know.

Eric