The pump makes a little noise when it's working to evacuate the system, but should become quiet once the system's vacuum is restored. Ideally, the system should hold a vacuum for about 24 hours. In that case, if the car is used regularly, you would rarely hear the pump.
If the system leaks down right away after the engine is switched off, then expect to hear the pump for a short while whenever the engine is started. If that's the case, you should check the entire system for loose connections, cracked hoses, diaphragm leaks, etc... fix any leaks. If the system is 'tight' but still leaks down, it may be the vacuum pump that is in the slow process of failing.
A temporary noisy period after each start-up is annoying (embarrassing for an exotic car to sound like that), but the system should still work once the vacuum is restored and the noise stops. No need to panic, but don't ignore it. Schedule a repair at your convenience.
If the pump makes noise all the time (sounds like a goose with nasal congestion) then there's either a significant leak in the system (a hose is disconnected somewhere / a diaphragm is torn or punctured), or the pump has died. The easy way to tell is to disconnect the vacuum hose from the pump, plug the pump's vacuum port, then start the engine. If the pump is quiet, then the leak is in the system. If the pump honks full time even when it's port is sealed, then the problem is in the pump... replace it.
There are small aftermarket filters available for the rocker cover vents on vintage American V8's... hotrod stuff. K&N makes 'em, along with other companies. Shop around a bit, and you'll find one that fits into the vacuum pump's vent hose. The hose is long to get the vent's exhaust noise far behind your ear. Sticking one of the small filters in the end of the hose acts like a muffler.
Regards,
Tim Engel