Gong sound when toilet flushes

have an issue that has recently popped up and wonder if I should just ignore it or if I need to have someone take a look at it. We have been in our home for 22 years and replaced our masterbath toilet with a Koehler toiler about 5 years ago. For about a week now, whenever we flush the toilet, we hear a noise (like a gong sound) under the bathroom floor near the tub. Is that a sign of trouble?

Thanks for your advice.

Joanna