Most likely it's the inlet valve on the WC cistern that's "bouncing" on its seat.
Easiest way out is to replace the valve. It may be the older ballvalve type with a plastic ball on the end of an arm, or the newer compact, plastic valve and float combined.
If that doesn't fix it then it's possibly mains pressure related "water hammer". You may need to fit an arrestor...........
It will not explode let alone injure you. As Builder has said it is a faulty valve or 'water hammer' . If this is a recent development it is most likedly a faulty valve. Stay calm and carry on !
Thank you fellas!
So it sounds like I better invest in a plumber then. Didn't want to call one unless I had to.
And thanks, Eddie, for reassuring me. I have to admit I didn't really think I was in danger, just my warped sense of humour.