Noise when coasting along

Hey all,

What might cause a car to make a very annoying whining / whirring sound when you’re letting the car coast with your foot off the gas. If you get back on the throttle, it goes away. Its only really an issue between say, 25-55 MPH.

Even if I shut the engine down the noise is still there.

I already mentioned this to my mechanic and he said not to worry about it unless it gets worse. When is it time to go back in the shop? It’s really just an annoying sound that I can live with, it doesn’t affect the way the car operates otherwise.

Its a 99Accord with say, 134,000, Auto, 4cyl. Thanks.