What is electric water tank noise?

I have a loud vibrating noise coming from my water tank which has occurred twice so far. The first time was after having a shower. The second time it occurred after running the water in the bathroom sink. I realised what it was the second time and looked at the water tank which was almost vibrating with the noise. I shut the stop cock which is underneath and that stopped the noise. Then opened the stop cock and it was ok. Any ideas why this is occurring and is it dangerous? What can I do about the problem?