Thumping sound when flushing the toilet

Ginfay

Just recently I've been hearing a thumping sound after I flush my toilet. Usually it happens when the tank is filling up and I am running my sink faucet at the same time. The thumping sound usually goes away after the toilet tank fills up. This happens with at least 2 or 3 of my toilets. The noise does not happen when I only flush the toilet and no other water is running.

I thought this might be a water hammer but according to another website, the hammering would only occur when the water is not running. (not sure if this is accurate or not).

Does anyone have any suggestions on what might be causing this loud thumping noise?

Thanks.