I would say that even if you're planning to use it at 4 a.m. it's unlikely to bother them unless it's an unusually noisy one. Besides, whoever worries about disturbing the UPstairs people :-)?

if it's a wood-frame 2-story building, vibrations may travel. Ditto noise, which may be amplified by the "sounding board" of the hardwood floor.

I don't have any experience, unfort.

They will hear it. Growing up we had a treadmill in our basement, concrete floor. I slept on the 2nd floor and could always hear my Mom walking on it at 8 am. Just running the treadmill it makes noise. Our model was probably bought in 1993

They can't evict you for having a treadmil. These days they have washers and dryers in all apts. They make the same amount of noise if not even louder than treadmills. I live in a townhouse on the 3rd floor loft. I put a black thick mat under my treadmill. My roommate below barely hears it. Sometimes if you put padding underneath of it it will block at some noise. Running children, animals, music are all the same are they going to tell you that you can't have children talking or playing or that you can't listen to music. Please give me a break. I think as long as your are working out at a reasonable hour after work, early evening even if the person does hear it will only last about up to an hour. In my opinion this is obsurd. I plan to move out on my own soon and move into an apt and I will be damned if I can't workout on my treadmill!

We have a state of the art fitness center on the property of our apt complex and so it states in the lease that any and all workout equipment is forbidden in the actual apts. I guess it depends on how strict your area is with noise codes. Most of us here have never given it a second thought since there is a wonderful workout facility for us to use. They have big screen tv's............tons if equipment........the room is glass so you can see the lake and pool if you don't want to watch the tv's. I guess there is no reason that anyone would complain if they provide that for the tenants?

"In my opinion this is obsurd. I plan to move out on my own soon and move into an apt and I will be damned if I can't workout on my treadmill!"
With this attitude I think you are heading for trouble. You should consider buying a house rather than renting, or at least, find a place where there are a bunch of health fanatics or young noisy people. Most laws imply that people are entitled to peaceful enjoyment of their apartment.
With things that are very common like washers and dryers it's hard for anyone to complain, but when you get into unusual noises people feel they have the right to complain. I think if it got into court, the judge would agree that it's not acceptable.

As someone who has a friend who lives under a guy with a rowing machine in his 2nd floor apt, I can PROMISE you that a washer and dryer are not even slightly comparable! We can sit in her living room and watch the light covers work loose on her ceiling fan when he is "rowing" but you can't hear the washer/dryer running at all.

I think anything like that should be for people who do not want to live with others in a commmon area. You have to be willing to give up things in order to maintain peace just as you would want if you were stuck under a treadmill.