Perhaps the aircraft levelled off after take-off, or because it was approaching cleared altitude, reduced rate of climb and throttled back into a gentle climb only, then maybe was cleared for a full power climb again?
Noise perception varies greatly in a cabin as you change position. Sitting just in front or abeam the engines is noisy, and to the rear you get a dull roar from the exhaust. people say the Airbus cabin is slightly quieter, and I think I agree, but it does make some strange sounds! The engines sound like an electric strimmer, which is one of the two reasons the Airbus is known as the strimmer, and that hydraulic power transfer unit under the floor of the rear fuselage makes a very weird noise on the ground- I actually identified it as a barking Alsatian dog!
But at the end of the day, they are 2 peas in a pod. Same size, same engines, same fuel consumption, do the same thing, go to the same places. It's just one is flown by pilots and one runs on French software!