lol its just on screen vibration. go to settings to fix

Maybe for you but it is actually a clicking/creaking sound and is different from the tik sound that comes when the system sound is on (and ya screen touch vibration is off). In my case it comes when pressing the top center part of the screen. Here, in one thread, I read it can be stopped by tightening the screws which I tried but failed to get rid of it. From my observation what I found is, after pressing and sliding around the area gently, the sound disappears but reappears after sometime which is clearly the sign of screen not being properly attached. If any hardware specialist can confirm that it won't get worst or affect the device in any way in the future, I think I can live with it or at least wait for the N900 to be available here so that I can take it to Nokia Care (or will they be able to fix it even though its not available here yet?).

Mine clicks in the top right corner too. No screen protectors ever used. How to get it fixed?

Surely it should be 'creaking' not cracking.