My wife makes that very same sound when i leave the seat up? lol
Paul|||open up wider the intake valve on the wall.|||I would say it is the fill-valve. Remove the tank lid and flush. You should be able to hear it better and find the source. If it is the valve, you can change it to a Fluidmaster 400A valve for about $6. The kit comes with really good instructions and is an easy fix.|||It's sounds like you ballcock is malfunctioning.
It may be normal, but if it fills up fast enough, I would think about installing a Korky ballcock.
Most us plumbers consider a normal fill up time within 30 seconds after flush.
If it's more than a minute, you may have a blockage in the water supply, or it may be partially open.
Some call a ballcock, a flush valve by the way.