Originally posted by Grimm
Disagree. As per my post above. (based on personal experience)
It really depends on how loud that car next to you is. Same for any noise.
Come on guys, you dont go completely deaf when your around a loud noise. Some of these answers are too absolute. You would think you couldnt hear a large explosion next to you if the engine was running.
If the sound is there, it can be heard, Just depends on how close and how loud it is before a human ear is going to notice it. How else do explain being able to use a head set in a real A/C 
Back to the original question, I dont know, The ony way to prove the answer is to have you guys buy me a P51 and a 109 and help me test it out
I think Grimm has a point here. (Though he may not know it.) It is like the people who live next to airports, busy highways, or Niagra falls (just an example.) After being exposed to a certain and near constant sound, you start to block it out.
Most people here should know that the sensation known as "hearing" is not just a physical thing. The movements of the bones in your ear must be processed cognitively before they are recognized as a particular "sound." This being said, sound waves and distances cannot be the sole judge of whether a person could actually hear a sound or not.
If you were a regular pilot of any given WW2 plane and heard the droning of your engine (or even "screaming sound,") You could bet you would notice a different sounding engine. Just as Grimm can tell the difference between the sound and pitch of his engine during a pass and that of his opponent.
One more example: If i shot off 10 firearms off different calibre at say....1/2 mile or almost any distance guys like lazs could tell me that they were the same firearm being discharged or a different one. He might not be able to tell me exactly what they were (though i fear he could,) but that they were at least different.
Once you becoming accustomed to anything sensory, anything new or different than what you are used to would stand out.
my 2 cents.
Btw lazs this wasnt a dig at you for being fond of firearms, just using you as an example of someone being familiar with a certain experience being able to notice things a little better than others.