There's a lot of variations in the astronauts' subjective impressions of the sound: "muffled roar", "gutteral roar", and allusions to infrasonic vibration, for which "rumble" might be a fair description.
Collins, in Carrying The Fire:
Trust your instruments, not your body, the modern pilot is always told, but this beast is best felt. Shake, rattle, and roll! Noise, yes, lots of it, but mostly motion, as we are thrown left and right against our straps in spasmodic little jerks. It is steering like crazy...
Jeffrey Kluger's Apollo 8, largely drawing on Lovell's experience:
The noise inside the cockpit was like nothing the astronauts' simulator training had remotely been able to reproduce. For at least ten seconds -- though to Anders it felt like the better part of a minute -- the crewmates had no way to communicate with one another...
Not sure why the situation would have changed after ten seconds -- maybe acoustic shock reflected from the ground? Once the vehicle went supersonic, the engine noise would only be transmitted to the crew through the structure, not the air, but that happens at about T+66.
Al Worden, Falling To Earth:
Inside, I heard hardly any noise: only a muffled roar far beneath me, as the engine thrust vibrated up through the rocket structure. We began a smooth, slow rise from the launchpad in an eerie kind of silence ... We quickly went supersonic; the engine noise could no longer reach us.
Tom Stafford, We Have Capture:
The ride on the S-1C first stage was smooth, nothing but a guttural roar you felt rather than heard.
Quite a dialogue among the crew in the Apollo 11 debriefing:
Armstrong: Concerning the noise/vibration intensity at lift-off, it was my impression that the combination was rather severe until approximately the time of "Tower clear", at which time there was a significant decrease.
Collins: Yes, but would you say noise? I would say vibrations. I thought the noise level was much less than I had expected. The vibration was more.
Aldrin: How about a rumbling? That is physically felt as much as heard.
Collins: You don't hear it in your ears. You feel it in your whole body. Whether that's noise or whether that's vibration, I don't know.
Armstrong: I would agree that the noise was low level.
Collins: In terms of interference with communications, though, I think you would also have to say that it is low level.