Running water sounds when all the taps are closed is ALWAYS bad news. Only exceptions I can think of are some RO and ice-maker installations in kitchens and self-cycling water softeners. In the bathroom....no. Strongly advise finding/indentifying. Strongly advise NOT letting it go no matter who told you it's "fine". (In addition to its being "fine", I would think he'd at least tell you what it is!) At best, your hearing's bad or there's water being wasted. At worst, you're about to discover a disaster.

Thanks! We had a leak when the toilet was flushed from the flushometer fitting. I was told the rubber ring went bad which was strange as it supposed to be new. I didn't hear the water noise then, but when it was fixed with a new ring, it was back again. Could there be any connection? Thanks!

I am not familiar with the device you've described so have no opinion. However, surely if there was nothing going on before and something going after I would suspect association. Wish I could be more help. In any event, do proceed to identify and evaluate. Running water sounds where there should be none should not be left alone.

Besides the sound, do you see any water moving in the bowl or tank? If you do, the flushing mechanism might need to be adjusted or replaced. When you hear the sound, go to where the water meter is and see if the meter is moving. If no water is being used in the house and the water meter is registering usage, you have a leak. Shut the water off to the toilet and see if the sound stops. Then go back to the meter and see if it's still registering usage. If it doesn't, call the plumber back, the job he did wasn't correct.
Ron
Look for the water movement before you turn off the water.

I don't think the lister, Blondelle, ever mentioned if another fixure in the house was being used when she heard the noise. If you are in there and something else in the house is turned on...a faucet, washer, dishwasher, etc. you might just be hearing either water running through a water line or a sewer pipe.
If you can hear it all the time and nothing else in the house is turned on, then you should be finding water running out somewhere. Since water always runs downhill, start by looking at the walls and ceilings below this bathroom and then in the cellar. If you don't see water then you know it's not a leak.
The exception might be another toilet in the house that is leaking into the sewer pipe and you won't see that unless you really look for it.