Whenever I sit on a toilet, I hear one bang sound. It's not from pipes. It sounds like the toilet hit something.

I tried to look the movement of the toilet when my weight pushes it down. The bottom part of the toilet moves a little bit, but really just 1mm. I don't know if this movement makes the sound and I don't know what it's hitting.

Funny thing is, there is no sound when I sit very very slowly.

No leaking, No sound when I wiggle the toilet.

There should be no movement at the base of the toilet so try to carefully tighten the two bolts on the base. Might consider replacing the wax ring while you are at it. However, your sound is more likely coming from between the tank and the toilet. Shut of the water and flush. Remove the tank lid and try to tighten the two screws inside the tank. You may need helkp[ tp hold the nuts on the bottom of the tank. Over tightening can be even worse than a banging noise, you can break the China so be gently with your throne.

Thank you for your advise.