Question of the year.........

I hope someone may be able to help please......

Throughout the day and mainly at night a 'tapping' noise comes from my 4
foot SW fish and reef tank.

The noise sounds like 'clicks'. about 3 or four in a couple of seconds.

It is 99% the same as tapping the glass heater against the side
gently.....(but it is not this)

It is hard to pin-point the area of the tank it comes from.

It is quite loud.

'click-click-click'......driving me mad.

It is not the star fish tapping or the clown fish or live rock rubbing or
coral swinging in the current.

all help appreciated.

lg