Speaking from the recent Marvel movies... I just watched Captain America last night (only missing the Hulk's individual movie now). I noticed that the HYDRA weapons made the same little "whine" before they fire, just like the hand repulsors built into the Iron Man suits.
Are the two technologies related at all, in the history of The Avengers universe or is there some other in-universe explanation cued by the use of the same sound? Maybe Tony Stark's designs were influenced by prior HYDRA work? Or was this more likely just the sound designer happened to like that sound effect and wanted to re-use it?