I think my neighbor heard me masturbating really loudly.
What made you believe that your neighbor heard you masturbating?
I think he made a facial expression while I was masturbating that showed he heard me while I had my headphones on.
It is completely inappropriate to masturbate while your neighbor is near you. You should only do that in private, because other people don't want to be involved in your sexual experiences without their consent.
He's in his house, my house is a part of his area, so he can listen to what I do. What I do in my house is none of his business.
Even though it is technically two separate houses, you should probably try to be a little quieter. I doubt that he wants to hear the sounds of you masturbating while he is in his home. While he is technically not around, he still kind of is since he can hear you.