Internal and external noise are both very real things. For me personally, I think internal noise is the most distracting. I tend to zone out in my head when I'm public speaking. This results in me rambling or forgetting what I'm talking about. I also get distracted if I'm hungry, have a headache, etc. I prevent this by having a good outline for my speech, and telling myself that I will do good before I present. Envisioning myself succeed before I present helps with the internal noise a lot, and is the method I use the most.
I agree that internal noise is more of a struggle. I loved how you talked about preparation and having a solid outline were helpful to you.