I took a short video with my iPhone to demonstrate:

As you can hear, the audio is a little exaggerated by the iPhone mic but it’s pretty obvious that my heavy footfalls are much louder than the sound of the motor (which I don’t really hear at all) or the deck itself. If you’re lighter than I am (and at my current weight of 298 lbs, you probably are) it will probably not even be as loud for you.

Something to note is that our treadmill is in the basement, sitting on a vinyl treadmill mat, on top of a carpet, on poured concrete. So your mileage may vary if you place it on a hardwood floor that isn’t the basement. I have no idea what it’ll sound like to your downstairs neighbors, but it’s not really any louder than I imagine it would be to walk on a laminated plywood board (which is what I believe is the material of which the deck is constructed).