i live in a 5 yr old building in north york.
i don't hear my next door neighbours at all, except for the infrequent hammering of a nail.
the door is definitely not sound proof in many newer buildings (i.e. such as mine). fortunately i live in a unit that's not near an elevator, trash chute, or have any neighbour's door across from me. i purposely looked for this type of unit position during my condo search.
i do not hear any background humming noise when it's dead quiet. There is some loud wind noise that seems to come from nowhere, but i attribute that to normal background noise or "building noises". I've gotten use to these background noises through time.
i do hear noise coming from upstairs such as items being dropped and sometimes muffled footsteps. but i would only hear these noises if it was dead quiet in my unit. However, there have been a few times when the foot steps are quite loud but fortunately they have been very infrequent.
i've been warned by many condo dwellers that noise is unavoidable and i have to learn to live with it (within reason of course). I'm pretty sensitive to noise so when i was condo shopping, i did the following:
1. Ensure your door is not located near noisy areas (as i mentioned above)
2. Monitored the traffic of a building (i.e. the lobby). Although not always foolproof but I assumed that the more young people i see, the more potential for noise.
3. while waiting in the lobby with my realtor i was lucky enough to ask an existing tenant about noise.
4. made sure the balcony did not face a major street. You do hear the noise when the balcony door is open no matter how high up!
5. this is a minor consideration but i also looked for a unit that did not have a shared balcony with a neighbour, i.e. separated by a glass divider. However, in my current place, the balcony is pretty secluded, but i can still hear the folks above and below me quite clearly.
6. read up on the condo board rules and regulations to see how noise problems are enforced.
Unfortunately, even following the above, there is very little you can do about what type of neighbours you may live next to. You just have to hope to high heaven that they are quiet. If not, hopefully there are measures you can take through your condo board/management that can resolve any noise issues.