Double vibrate every 2-3 minutes

[Blind Freddy]

My Moto G does a double vibrate every 2-3 minutes..brr ..brr. It will start doing this every now and then, for several days/weeks, and then stop, for no reason I can work out. It does it long enough at each "session" of several days/weeks to get on my pip. It started again a couple of days ago and I have no idea why. It shows no sign of notifying me of anything, I have no emails, I have no messages...etc.

Does anyone know of some known cause of this irritation?

Thanks for any help. I know it's a foggy question, but as I said I have nbo idea....

Nick