Lynn from Cambridge, Ontario asked about a horrible rattling sound in their plumbing pipes when someone is using the outside hose bib and someone flushes a toilet. Only when those two things happen at the same time. What is it?
Actually it is quite simple and you can understand it best if you blow air out of your mouth right now and slowly close your lips until you get a nice loud whistle. When your lips get to just the right place with just the right air pressure coming out, they vibrate and create that sound.
In this viewer's house, there is most probably a loose washer in the hose bib, as you can see in the photo. With just the garden hose on, the pressure is not just right. But with the faucet open to just the right place, and the water pressure reduced by flushing the toilet, the flow of water arrives at a perfect speed and pressure for just a minute. That little washer is vibrating in harmony with the pipes, creating a much exaggerated noise, like in any musical instrument.
We rarely change the washers in the hose bib, because we don't notice the drip. But an old hardened and shrunken washer can get loose, and vibrate like a symphonic orchestra. A 30 cent solution -- change the washer and tighten down its holding screw.