My iMac 27" has just started my a ticking noise

My iMac 27" has just started my a ticking noise for the first few mons of start-up. The noise comes from the bottom left-hand corner. If I squeeze the base it gets louder while I'm holding it.

Sometimes it goes after a few mons of veing started up or woken up. At the moment it can still be heard, just at a very quiet level.

It did start after installing the OSX update but I'm guessing its a fan issue?