I would argue that since the spell is physical in nature, it affects the physics around the detonation point. Basically, all the spell is doing is creating a tremendous amount of friction in a single small space, then letting it go. So while casting the spell would certainly be silent, and the initial implementation of the spell would be silent, everything from the nanosecond after casting would make noise. Though, if it was in a vacuum... hmm, I wonder if it would even work? Even space isn't a true vacuum, but it's enough of a vacuum that there is no likelihood of creating a shock wave. I'd argue that in space, no one can hear you Blast. And no one would feel it either.