Thats normal.
Remember to take into account that, when you are crouched, you are closer to the weapon (Rocket Launcher in this case) And if you are closer to it, chances are the sound will be louder.
In other words, this isn't a problem at all, the only reason why no one else has replied is because everyone experiences this, its basic sound dynamics.
Summary: The closer you are to the sounds source, the louder the sound will be EVEN IN BLOCKLAND