Get the blue waves you want in the middle of the screen using the zoom, play, scan, etc. tools, and Stop. The tools won’t work from Pause.
Click the blue wave and hold, drag sideways, let go. No keys. The area you selected should get darker, and you should be able to play just that selected portion without the play point spilling over into the rest of the show. You can now use that for the profile step in Noise Reduction.
Noise Reduction only works on noises that never change, like air conditioning noise, computer fan noises, microphone hiss or hum. It doesn’t work on traffic noises, music, airplanes, conversations, radios, etc. When you turn Noise Reduction loose on your show, it’s going to go ripping down the waves looking for exactly what you put in the profile. If you’re trying to get rid of freeway noises, sometimes you can luck out and the roar of the roadway will stay constant long enough for Noise Reduction to grab it, but that doesn’t work with trucks driving by one at a time or an occasional car.
The running joke is to not get any voice in the profile by accident or Noise Reduction is going to try and get rid of the performer.