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Unfortunately the D90 doesn't have a Quiet mode. The only cameras that can be used completely silently are mirrorless cameras. All DSLRs will have to move the mirror out of the way just before the shutter opens and then bring it back in down position. Now some models do have what they call a Quiet mode, but it's not noiseless. They only have a damping system that slows the mirror movement so it sound a bit less. But if you try to shoot in High Speed continuous mode, most DSLRs will make some noise.