We just have noticed that after we flush the toilet in the bathroom located downstairs, after it refills the tank, there is a loud thump noise that ends the water cycle. Again, this was not happening before and now all of a sudden, the noise has began. What could this possibly be? What should we check for?

Thanks in advance!

Sounds like your air chamber has lost its charge, the thump is the water hammer when the fill shuts off. Shut off your main and open some faucets to blow off pressure. Open the connection at the stop beneath the sink and allow the pipe coming out of the wall to drain, then reconnect and open the main. This should have allowed a cushion of air back into your chamber.