The MP5SD is the integrally suppressed version of the famous MP5 submachine gun. This video, taken by the THOR Group, shows just how silent the gun is.
Unlike most videos on YouTube the sound on this video is very clear and you can distinctively hear that a lot of the noise is caused by the bolt itself, rather than the escaping gas.
One of the reasons I love my suppressed .22 bolt gun is that there is no noise from the action other than the firing pin smashing the case rim.
Thanks to Max for the link.