Besides above, I have read that you can be having a nerve ending issue and an over stimulation of the nerves. Things that might trigger it are sitting too long, riding a bike, masturbating too hard. Things along those lines. If this continues, you may ask your doctor if you need to see a neurologist. I also read, believe it or not, stimulant related drinks like energy drinks or coffee even can add to the issue. So, I'd see your doctor to discuss ruling out a UTI, and then some of these other things. Let us know.
Is your whole penis vibrating, or just the head, just the shaft? Any burning when you pee?
Men sometimes complain of this when they have a urinary tract infection, but you'll need to see a doctor to make sure that's what it is. Don't try to treat it yourself. You could also be dehydrated, so try drinking more water, and laying off caffeine and alcohol.
If it continues for more than a couple of days, definitely see your doctor.