As soon as the toilet flushes, the tank is refilling and about 3-4 sec into that this loud humming starts and lasts about 6 seconds. Which by that time the tank is full. This is only on the 1 toilet downstairs. All other toilets work fine. O other item that uses water is effected by this humming in the house. Any ideas?

Lift the toilet tank led check inside and tell us which fill valve do you have, see the images below, Thanks.

John

The one with colors.

Great, Close the angle valve under the tank flush the toilet, remove the screws on top of the fill valve, remove the cover and the ballcock, remove the seal, put you hand on top of the fill valve open the angle valve for a second to flush out the fill valve, inspect the seal for any damage if OK turn it over and assemble the fill valve, hope this will.

Thanks
John