Cool Cat noise level

[Big Vic]

My A128S Cool Cat is installed so that there is a large enclosed empty space next to it. This empty air enclosure amplifies some of the noise and vibration coming from the unit as it runs.

I filled this empty space with a folded old foam mattress pad, which greatly decreased the amplified noise from the unit, but then I realized that I might be impeding air flow to the coils.

My question is how much of that space can safely be filled with some noise damping material versus how much space should be left open in order to allow adequate air flow?

[Jacink]

I got a hot water heater fiberglass insulation and tape it to the bottom of the floor all around the cool cat and to the plywood cover on top