Just for reference, I downloaded a decibel meter to check mine. My 48", 1200 cfm, 10" duct range hood without (!) a remote motor reads 56-58 dB when on its lowest setting, and standing about a foot from it. There must be something wrong with your install or the hood itself. I had to turn it on the 3rd setting (quite powerful, 3 of 5) to get to 70dB, and my hood is 1ft larger.

10-inch duct for 1200 nominal CFM (probably actually 800 -- 900 depending on MUA.) After growing the duct, put a Fantech silencer between the hood and blower. (Note 10-inch silencer is 14 inches in diameter.) You should be hearing baffle hiss once silenced and duct upgraded.

A few thoughts...

- Many noise apps are not very accurate. Most accurate we've found in our lab is Decibel X on an iPhone 8 or newer. Make sure you are using dB(a), not dB(c/z), are 1m from the hood, etc. More on how to accurately measure: https://www.houzz.com/discussions/6040827/range-hood-noise-project

- Duct and blower should be mounted with strap and NOT ever come in contact with framing or other elements that transmit vibration noise and effectively turn your house in to a giant speaker.

- Yes on duct possibly being too small. If so then this will result in increased noise.

- Bends/Turns (w/ proper sized duct) are actually good for noise reduction even if not so good for overall performance.

- Distance to blower is good for noise reduction.

- A duct silencer between the hood and blower can be very helpful.

- 1200 CFM is a lot for such a small hood. You likely have a lot of noise just trying to pull a lot of air through a small baffle.

- Baffle design has a big impact on noise levels. Many consumer hoods do poorly on this. How loud is it w/ baffles removed?

- Your hood lacks sufficient containment volume which requires you to operate at much higher CFM's (and thus greater noise) for the same (or worse) effectiveness at effluent removal.

- Blower may be out of balance.

Not necessarily noise related:

- Your hood looks the same width as your range. The aperture/opening s/b at least 6" wider than your range and ideally at least 27" deep.

I am experiencing a similar issue. The fan/blower is virtually silent without the baffles installed. With the baffles installed, there is a high-pitched whining/whistling caused by the air moving over and through them. A customer review on Home Depot mentioned that Zline told him to turn the baffles over. I did that, and the whistling is gone, but the air noise is still quite loud. Unfortunately, I think it's just incredibly poor design on Zline's part.

Does anyone know if I could buy replacement baffles from another vendor that would fit the Zline hood?

"Flame Gard" www.flamegardusa.com makes various baffles for commercial systems, but fit might be a matter of luck. You can take your baffles' dimensions and look at Flame Gard's product lines. I am sure that there are also other commercial sources.

ModernAire makes custom hoods, so it is possible that they could make custom baffle assemblies.

I think baffle hiss is inevitable with an affordable baffle array. One cannot force two reversals of direction with simple shapes and expect no turbulence.

Imagine this: Company A management tells design/drafting (not necessarily in-house) to draw up an easily manufactured baffle assembly that looks like all the other baffle assemblies one might find images of. Company B engineering tells design/drafting to draw up a test baffle assembly capable of taking different baffle shapes and offsets such that noise, pressure loss, and grease extraction efficiency can be measured and optimized. The only constraint is that the baffles be amenable to fabrication by stamping. Ultimately, which approach is more likely to achieve customers' performance expectations? Which one will have to be more expensive to pay for the higher development cost.

The baffles (and entire system) on our Accurex are silent at ≈500 CFM. Nearlly silent up to about 800 CFM and I think at 1400 is still much quieter than consumer hoods on low.

I think a big problem is that the consumer companies simply don't want to invest in engineering to produce a quality product because American consumers don't generally care about quality.

Companies selling commercial hoods know that if they don't produce a product that is both quiet and high performing (and easily cleanable in a dishwasher) that they'll not be able to sell any because their customers care very much about these things.

The same thing is prevelent across products. You cannot buy a well designed sauna in the U.S. and the saunas sold in the U.S. aren't marketed elsewhere because consumers elsewhere demand proper saunas and won't touch what is sold in the U.S. The average coffee (or any food) sold in the U.S. is quite poor compared to what is sold elsewhere. Same for the build quality, particularly energy efficiency, of homes built in the U.S. And on and on.