Making ventilation more stealthy

I built a cabinet in the basement and it's not vented to the outside (only have one place I could vent it and it would not be discreet), dimensions 6.5ft tall x 4 ft wide x 3ft deep.

One 400w hid light, non-cooled hood. 6in CAN inline fan and a CAN33 carbon filter connected with plain flexible ducting. Good sized passive intake, small circulating fan.

I have a fan speed control but at it's lowest setting it's still not very low.

The cabinet sounds like an idling rocket ship. I think most of the noise is coming out of the exhaust and bouncing off the block wall.

Does anyone have any suggestions? My initial thought is to get a different speed control along with a cooled reflector so I can perhaps dial the speed down some more.