The Blackhawks were the silent stealth choppers while the much larger Chinooks with dual rotors were MUCH louder.
The local who heard the choppers stated that they were much louder than normal so it was probably the Chinooks that he heard and not the stealth Blackhawks. Based on that statement it would seem that the locals were accustomed to hearing helicopters flying in the area but not at night.
Osama was described as being somewhat dazed and confused when the SEALS broke into his room so it seems probable that either he didn't hear the initial assault or he was accustomed to hearing choppers and gunfire in the neighborhood and he didn't wake up from the noise until the SEALS started blowing things up to get past the barricades.
Wow. What you said explains clearly a lot of things. Thanks.