Buzzing in rectum after radiation

Sounds like mild radiation proctitis — I have that too. It's very common, so I suspect it was just the "buzzing" description that threw off your onco. 🙂. If you call it "mild intermittent rectal irritation" it will probably track.

Which did you have —> SBRT, IMRT, or Proton radiation? Did you use a rectal spacer? (I didn’t experience any rectal issues.)

I did 5 & 1/2 weeks of hypofractioned radiation treatment. I had SpaceOar gel inserted.

That's an interesting way to describe it but kind of accurate. For about 2-3 months after finishing SBRT I'd have all kinds of weird nerve sensations in that general area. None were painful (or pleasurable for that matter)--just odd. Things settled down after that.

You can see if it gets more frequent with irritants like spicy food. And stay away from fibre supplements like Metamucil, because the psyllium fibre can scratch the damaged area and increase the irritation.

Did your radiation oncologist prescribe any type of suppository for treatment?

If you wish to use mild and vitamin based suppository that can calm and lubricate the area you may try "Key-E Suppositories" based on coconut oil, palm oil end vitamin E. There are suppositories that contain cortisone and need to be prescribed but they can make area even more sensitive and thinner if used for longer than 2 weeks. I never personally had radiation induced protections, but I had some "issues" and found those Vit E suppositories useful. Hope they help.
PS: I also think that your description of the sensation is very good. Irritation can also cause nerve endings to act up and make that "buzzing" feeling. Some doctors are just very limited in anything else but one thing that they do , unfortunately.

Was it followed by the rapid Penguin waddle to the bathroom?

You are quite right about doctors. When I described that sensation to my RO he said he never had a patient describe that to him.

No, fortunately! Had zero bowel issues and fairly minimal urinary issues. My main post-treatment issue is that the ADT is just not wearing off and my T keeps dropping instead of going up.