We have an old fashioned flushometer, replaced about 3-4 years ago.For the last month it produces a very loud thud near the end of a flush.

Valve closing to fast, or a loose pipe knocking against a joist or structure. Depends on exactly where you hear the noise. If from the unit you may be able to adjust the water flow, or just replace the valve, check diaphragm, or clean/replace the bypass. The simplest test is slowly adjusting water flow, and see if that quiets the thud.

https://www.youtube.com/watch?v=Upkq8qIwD5M

Or you have a water hammer condition in which case it gets more complicated.

https://www.youtube.com/watch?v=lpT_-3HR5yo

What do you call "flushmeter" ? Do you have toilet without tank ?

Milo