I have no idea why they make a noise so I think first talk to the company you got them from to see if there is some way to fix this.Do you have hydronic heat everywhere? I would contact Slant-fin .

I have one called the ”Quiet One”. It is 16 years old and makes a gentle fan noise which you can’t hear outside of the room when it’s on low speed. Hi speed it’s a gentle fan noise but you can here it in the next room. Slantfin is a good company so I am sure ther is something wrong with the install or the particular heater(s) but it does seem a little odd that is both of them . i would contact your installer first And if you don’t get anywhere with call Slanfin. The noise is not because they are hydronic but because there is an issue that should be fixable.