There was a chap at Warminster a week ago who had clearly modded the hell out of his AEG to make it as quiet as possible, it only made a dull thunk sound but I can't recall what it was. I think decibels makes a difference but it's also the tone, from that video the tone was a very distinctive AEG sound whereas a more muffled deeper tone like his seems to be more difficult to recognise.
Stealth is on my to-do list of mods for the future.