Yes...that's the normal sound the pump makes
And it can be a real thrill in the middle of the night if it's under your berth as mine was.It should only run for a minute or less after each flush, to establish suction (vacuum)for the next flush.The pump actually does two things at once. The suction only pulls bowl contents as far the pump...the pump then pushes it the rest of the way to the holding tank (not to be confused with the vacuum tank) while it also pulls air out of the system between the bowl and the pump. A sensor in the vacuum switch--located at the end of the vacuum tank--turns the pump off when the right amount of vacuum has been achieved. SeaLand's claim that the V/Flush can use "as little as" 1 pint of flush water is VERY carefully worded. If only urine is flushed...no water added to the bowl first, no TP either...then yes, you CAN get away with that little, at least for a few flushes. But if you do much of that, unless you want odor, at least once a day you need to run at least half a bowl of clean water through it to rinse out the system. It's also advisable to add at least half a bowl of water (lifting the pedal adds water to the bowl without flushing the toilet) ahead of solids or any TP (iow, every time a female uses the toilet)...that's a quart or more. Stepping on the pedal not only opens the dome in the bottom of the bowl, it also "turns on" the flush water...so it's ESSENTIAL to leave the pedal down for at least 7-10 seconds after the bowl is empty to rinse out the pump and duckbills--to prevent a buildup in the pump or bits of waste or TP from becoming stuck in a duckbill, creating one of those pesky air leaks that causes the pump to cycle for no reason...at least another quart. And if you don't want permeated hoses, it's a very good idea--last thing before the boat will sit (or at least once a week if you're living aboard)--to fill the bowl to the rim with clean water and flush it through to thoroughly rinse out the vacuum tank, hoses and pump...'cuz suction splatters waste all over 'em and the flush water flow isn't sufficient to completely fill the hoses. So, averaged out over a week, the VacuFlush actually NEEDS about the same amount of flush water that most other toilets made today need: about 2 quarts/flush. If you already have a VacuFlush and you're using much less than that, you're asking for problems, 'cuz 90% of problems with it are the result of using too little water.