I would say that internal noise distracts me the most. I can very easily go down different rabbit trails in my brain, so it makes it really hard for me to stay focused when someone is lecturing to me (especially when it's something I don't care too much about). In my brain I would rather think about anything that interests me rather than listening to someone talk about something that doesn't relate to my life. Things that I try to do to stay focused is repeating what someone is saying in my brain. By doing that, I'm completely focused on what the other person is saying and I'm filling my thoughts with their words instead of my own.
I completely agree that my mind also goes down different rabbit holes of things I am thinking about, while I am trying to present or listen to someone present!
My mind can also go down lots of different rabbit trails during speeches! I've never thought about repeating what someone is saying in my brain, that's a great idea!