Water tank tapping noise

I have noticed when my hot water tank turns on in the morning (it’s on a timer for 0630) there is a distinct tapping noise (sounds like it’s dripping on the ceiling). The water tank is in the loft above the bedroom so I hear it every morning. I can confirm there’s no drips on the ceiling to suggest it’s leaking and I have been up there with a torch and can’t see any obvious signs of a leak. Any ideas what it might be?