He never said anything about being under acceleration. He did say that, at first, it needed to be in gear but that wasn't necessary later. Remember this:
"It used to be that it took a lot to hear this rattle, and I couldnt hear it if i was in neutral, only when it was in gear. For the most part, I still need to be in gear to hear it."
Did you get the 'for the most part' part?
Sounds like his machanic was a moron. Did you see the other things his 'mechanic' said?
It must be the fact that no nothing 'experts' keep trying to give poor advice. Surely you see that you fit nicely into that category.
Look son, you are wrong, you could admit that or continue looking like a fool.
Do you remember cars without knock sensors, fuel injection, computers and air flow sensors? I do and I remember turning the distributor (do you know what that is) and advancing the timing when I had premium fuel in it to see the power difference. There were no computers and knock sensors that did that back then. Grow up a bit and come back when you have some experience to back up what you like to tell.