Car whistles when not on the accelerator

That describes a vacuum leak almost perfectly. When you are not stepping on the throttle the vacuum level is at its highest and the whistle will be loudest. Should not be hard to find it. Take a short length of hose (water hose works well) hold one end up tight to your ear and move the other end around and listen for the whistle to get louder. When it is loudest you are right near the problem area. Then it is time to look real close for a leak. You can also use a spray bottle with water in it to spray in that area and listen for it to be drawn into the engine.
Then you replace the cracked hose or failed gasket to remove the whistle.

It would depend on if the leak is causing other problems or tests not to run. But I would find it and fix it ASAP because it could cause other problems if left alone.