I have one toilet that for some reason, makes a loud whistling sound when I flush it. It starts to make this noise when the flapper drops down and the tank starts to fill up. This noise just started 3 days ago.

I have one toilet that for some reason, makes a loud whistling sound when I flush it. It starts to make this noise when the flapper drops down and the tank starts to fill up. This noise just started 3 days ago.

And I'll bet that you have a regular float ball ballcock, (see image).
Shut the water off and remove the three screws on top . Pull the stem,washer and float assembly up. Pry out the washer and reverse it seating it firmly in place. Before you button things up place a pan over the open ballcock and turn the eater back on to flush out the supply. Reassemble and give it a flush. Noise all gone now? Regards, Tom

Speedball, Actually, I have a Fluidmaster set-up in the tank although I am not sure how old it is. I replaced the shut off valve and water line (steel braid) to the bottom of the tank about 2 months ago (the previous valve was junk put in by the builder). Any ideas?

Speedball, Actually, I have a Fluidmaster set-up in the tank although I am not sure how old it is. I replaced the shut off valve and water line (steel braid) to the bottom of the tank about 2 months ago (the previous valve was junk put in by the builder). Any ideas?

OK! I was wrong. Most noise complaints come from the other type. Same general principles apply here. Shut the water off to the tank and pull the cap off the top of the ballcock and twist the top off the ballcock exposing the lever mechanism, (see images) pull the nseal out, place a pan over the ball cock and turn the water back on to flush out the ballcock. Reassemble and test, hope things go better this time. Regards, tom

Speedball, YOu were right on the money. I followed the steps you described and the noise is gone. Thanks!!