There is a whistling sound coming from the plumbing stack on the roof. It's not the wind. Any ideas on what is causing it?

Is that whistle continuous or only when somebody uses bathroom?

Back to you

Milo

It is constant.

When you put your ear to the vent, you hear a whistle, yes? Turn off the toilets at the stops and listen for the sound. Still hear it? There is no real reason for the sound except wind or a leak being transmitted to the vent. Have any outside faucets turned on? Equipped with vacuum breakers? Mine make a whistling sound when left open to the garden hose without even running water through them when someone uses the house water.

Whistle is produced by wind passing through some kind of obstruction. In your case, I would look into the vent on top of the roof to see if it is clear of debris. While there, you can take garden house, put it inside the vent, and run water for couple of minutes to flush down what ever is inside. Also, you can put nylon sock over the vent to prevent wind from blowing inside the vent. Go back to your house and see if the whistle stopped.

Other than that, and as indicated in above post, sound may alos be generated by some device attached to your plumbing ( angle stop, garden hose valve, sprinkler valve, toilet valve, etc ). You have to investigate and eliminate each device one by one.

Back to you

Milo

Thank you guys. I have been doing some exploring and I think something is obstructing the vent itself... It had no cap on it. I couldn't reach anything before I lost light so I'll try again another day.

Thanks again!

Even a slight breeze can cause this Definition Bernoulli Principle (http://weirdsciencekids.com/Definitionberouilliprinciple.html)
Its a it like blowing across the top of a pop bottle.

Do you have a heating / ac system in the attic?

Do you have a heating / ac system in the attic?

No they are in the basement.

Does condensate tube drain to house plumbing? Is furnace running when you hear whistling?