The most likely thing is a leaking toilet refill valve. Shut off the water to the toilet and see if it goes away. Fill valves are cheap and easily installed if it is a fairly normal toilet - less than $10. Get a Korky Quietfill or a Fluidmaster 400a if you isolate it to that. While you are playing with it, it may be time to change the flapper valve, too. Depending on what is in there, sometimes it is as simple as replacing the seal...what brand of refill valve is in there?