They do make a racket. Running a U6 just under 6 cu m/hr a makes a racket. They can obviously wear out quicker, but the makers say it is OK for many, many years and years. A few years back, in a largish house with gas only for heating I recommended two 25kW Intergas Rapid combis. They consumed 3.02 cu metres/hr when full on. Two is 6.04 cu m/hr. A tad over, but it would only be drawing this volume when two showers are on full. Both were aerated rainfall showers at 9 l/min. The heating was turned down on both combis. so when both on full heat they would be both below the limit. The likelihood of full gas consumption was very slight. In this case the rare slight overload is not a problem.
If my memory is right, the Intergas can be tuned to be be just below 3 cu m/hr. Cue Intergas Plantpots.