Like, ya know, when that thing that makes you move, it has pistons and things, When your thingamajigy is providing power, you do not hear other peoples thingamajig when they are providing power. HiTech
Dunno if this was the sound of the A2A WGr21 and/or the 28cm's the SDK251 carries. I bet it's one of the things Lucas was thinking of when he chose the sound for Tie Fighters though.