If you are close enough to it when it goes off, it sounds like a giant fart.
One of the interesting parts of the Vulcan mini-gun is that it's all mechanical and interconnected. The parts that make it go bang are connected to the parts that make the barrels spin. If it's loaded and you walk up to it and rotate the barrels by hand, it will go off. To make it shoot, they just crank up the motor to spin the barrels and everything else happens automatically.
I was a helicopter crewchief in the US Army on a scout helicopter and we used to fly hunter-killer (pink) teams with the Cobra attack helicopter. During live fire training, we gave those guys plenty of room. They also shoot TOW missles, 2.75 inch rockets and a sometimes a 40mm cannon.
Occasionally, they would have problems with the gun and they would leave the gun installed (they took them in and out) so they could work on it. We shared a parking area with a company of gunships and we all parked facing the same way (into the wind) in two rows. If the wind was blowing from one direction, we faced their tails, if the wind was blowing the other way, they faced our tails. A couple of times, I'd be working on my helicopter and some of the gunship mechanics would come out and start tearing apart a Cobra. We'd wave (friendly rivarly between companies, but the mechanics would often lend each other tools and stuff...it was a long walk across the tarmac back to the hangar) and holler at each other. They are busy, you are busy...everybody is minding their own business. Suddenly you realize the "snake" (Cobra) still has it's minigun installed and those guys are working on it. We'd stop what we are doing, walk way out to the side and go over and say hello. We go over..."Hey how's it going?"..."Good"..."Gun not working?"..."No, not sure what's wrong with it?"...."It's not loaded, is it?"..."Nah, it ain't loaded"....."Mind spinning the barrels for me?"...click, click, click..."Thanks, man".....they would look at you and smile..."No problem".
One time the gun was jammed and the barrels wouldn't spin. We too a "break" and walked back to the hangar. The platoon sgt asks us "Got that tail rotor off yet?"..."No, we're taking a break. Guys behind us are working on a jammed mini-gun"...
About once a year, an "unloaded" mini-gun would crank off 3 or 4 rounds when it was being serviced. I never heard about anybody getting shot, but I wasn't taking any chances.