Don't you think your exhaust is a little big? 1000 CFM for 800 CF?
I understand carbon scrubbers slow it down some but you are talking about exchanging the air in there every minute you run that fan.

I think thats why your intake whistles. you are moving a lot of air very quickly.
If you can adjust the fan speed down to say half and you would still exhaust the entire room in 3 minutes.

I would say if you want to use that fan you are going to need at least one more passive 6 in with dampener for intake.
get a variable speed attachment if you cant adjust the fan speed already and turn it down.

your exhaust is up top so you don't need dampener because CO2 is heaver then air. Run the lights and thermostat and time the cycles. see how long it takes for those lights to heat up the room and what your max is. Thats how much time you have for 1500 ppm shot. From what you have described that is not going to be very long.

You might have to consider just a slow release with a constant flow through the room to keep temps right.
I did that in a cabinet grow where It always got hot.

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Yes, the CFM is higher than most people use. My logic here is that the quicker i can cool down the temp the longer the CO2 uptake period will be. If it takes a long time to bring temps down and the lights heat it back up rather quickly then that doesn't leave much time for CO2.

Higher CFM = Faster air exchange = shorter cooling period = longer CO2 uptake period

I think that I am going to cut the hole wider to 8" diameter and install an 8" Vortex 750 CFM with a backdraft damper. That would remove any whistle and lessen the negative pressure significantly. Another perk to installing a fan is that I can run ducting from the fan to the outside of the house to bring in colder air. The temp of the subfloor air is on average 5 degrees warmer that outside air from 8 pm till about 3 am. At around 3 am they seem to equal out.

That's kinda what i was thinking.

I would like to avoid a constant exchange for CO2 purposes but we'll see after further testing. Thanks a million for your feedback!

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No matter if you have passive or active intake, the whistling sound is from the movement of air. If you move air even faster, it will whistle even more. In my case, I cut a ten inch hole, and it works great for my purposes. Since you're aiming to use Co2 though, what you're going after is not the same goal as mine. My aim is to maintain a constant temperature whereas yours is to actually bring down the temperature enough to have actual swings in temperature reduction.

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True. I'm expecting that a larger intake and a fan will eliminate most of the whistling mostly because I have a Vortex 8" on one of my set of lights for cooling and it doesn't make nearly as much noise. We'll find out later today.

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Slowly but surely I'm learning to listen. Sorry for the stubborn streak. I cut an 8" hole and installed an 8" backdraft damper and ta-dah! no more whistling and the negative pressure is greatly reduced. You can lead a horse to water but you can't make him drink. Thanks, Mcgician, for leading the horse to water.

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see journal for update on cooling/ventilation system: First Grow - 3000w - Ebb & Gro - H&G - Sealed

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