If you have requests, they should be on your reservation. You cannot make requests at online check-in.
Online check-in allows you to indicate a time of arrival, so they can try to give you an available room(just might not be the one you are looking for), and place a cc on account for charging.
You need to call CRO and have any requests attached to your reservation. Do not wait; ressies are rolled over to the resort about 3 - 6 days before arrival, when room assignments are done.
If you have more than one request, I would forgo online check-in. They will give you the first available room and it may not be to your liking. To have them look for another room and have to re-run your cards takes time and sort-of defeats the purpose. Therefore, it is best to forgo online check-in.
Regarding requesting floors. You have to remember that while pretty much everyone who checks into a motel will have a car, many people have no transportation at Disney. They rely solely on ME and transportation on-site. The parking lot sides can actually be very, very quiet.
At the mods, you will hear the toilets flushing on any floor; they have the flushes almost like you hear on cruise lines. They are very fast though.
Other than that, unless you have a person bouncing a basketball on the floor above you or someone has the TV on too loud, you don't usually hear very much above or below you. It is only next to you.
Now, some people do request higher floors, not so much for the noise. It is more for the "critters."