Actually most NVG's do make a noise, however, it is only a very very quiet whine/whistle, and is easily over-powered by the slightest of noises (wind, walking etc)
The "power up" noise is made by the electrical circuitry and the actual light intensifiers charging up when the unit is first switched on, and is often [read allways] overdone in games.
The only way you'd really hear them is if you were stood still, listening carefully for the noise, in a quiet room. So not really a good application in OFP.
[Edit] Damn spellings [/Edit]