Whistling coming from overflow hole

[nobody]

Just did a wc anbd now i have a whistling sound(sounds like a bird) coming from the over flow pipe that has the hole on the top. 1st time this has happened. Any suggestions?

 

[Reef.]

Is there a tune to the whistling? If any tune at all post to YouTube it’ll go viral...or try adding a little more water, removing some water, changing the flow rate on your return or cleaning the return, may help.