turn off water flush toilet.....

does noise still occur? if so its a drain issue

if not its fresh water tank issue.

this narrows down the noise.

perhaps the tank water level is a bit above the dip tube?

of the flush valve is failing?

that happened here wondered what was going on, then part fatigued off
flush valve.....

flush valve may have been damaged when toilet was pulled the plastic
ones are cheap materials.