If you click "Finish order & export" while the develery is going, it makes "rocket start" sound again, and it's making this sound every 30-40 secs from now on.
You can click the 3D sound button above the launchers to mute the rocket sounds, or you can also click the mute SFX button to mute the rocket sounds, I will look into this issue, although it sounds like to me you are hearing the Space Probe launcher. Thanks for the comment/feedback/bug report :D
After playing about an hour I can easily say, that it's one of better idle games I played in a while, it blows away those "idle" games where you have to reset you progress to get anywhere further, and it gives you a goal (chapters) that you want to accomplish 5/5 from me !
 "Idle Empire 3"){kind=icon}
 "Idle Potato 2"){kind=icon}
 "Coins"){kind=icon}
 "Bear in Super Action Adventure"){kind=icon}
 "The Awakening RPG"){kind=icon}
 "AdVenture Capitalist"){kind=icon}
 "MARDEK RPG: Chapter 2"){kind=icon}
Fixed, found out where was this bug - check update notes for details.