For millions of years the aliens had been happy with Earth, which was one of the quieter planets. They had gone about their business farming its silence for as long as they could remember; extracting its inexhaustible calm and shipping it to the rowdier spots of other galaxies. The noise duly muffled, the aliens were able to rest and concentrate their brainpower on more important things, like expansion.
But then the inevitable happened. On a routine decibel-gauging trip, one of the aliens dropped its lunch and before they knew it the place was teeming with lifeforms. It was bearable for a while, the insects and the reptiles didn’t make too much noise, and they couldn’t hear all the stuff in the water but then the apes happened. Oh the apes and their vocal tracts. The quality of the quiet declined sharply, Earth was no longer exploitable.
So the aliens got used to taking the saucer out every now and then and giving the place a good dusting. After a while though the lifeforms showed resistance, so they had to up the doses and even try out a new formula that caused heated debate. It wasn’t a nice product, the lifeforms writhed a lot before dying.
It was quieter though.