Got to agree with Johnnie as after hours it's pretty quiet. The occasional ambulance on the highway will break the silence but definitely won't keep you awake at night. The further up Walker Street you get towards St Leonards Park the closer Walker St is to the the Freeway so you do get some background noise but nothing to complain about. The location trumps it all!
It depends. I stayed at a top floor apartment in North Sydney where you're far from the streets below and can't hear anything.