Hear a hollow sound when the toilet is flushed like water going into cave.
Any ideas on why and how to repair? Thanks.

Did the toilet always makes a sound or did it just start? There are choosing to check. Checked the tank level at the ball cock shuts off. The level should be a half an inch below the top of white overfllow tube. Another thing you might check is the small jet Hole located in the bottom of the bowl. Let me know if that helps. Good luck, Tom