When we flush the upstairs toilet it roars like a jet airplane taking off. The sound/vibration is clearly coming from the water supply intake. We can hear it and feel it in the pipes downstairs. The problem seems to have begun a couple of weeks ago when I turned off the water at the toilet to clean the thing. When I turned it on again - the roaring began every time I flush.

There is probably a restriction at that valve, corrosion or gunk build-up. After having moved the valve, make sure it's fully opend, but not "jammed" open.

Thanks "Captain Rich",

I made sure it is open all the way (it is) and flushed a few times.
The roar is still there. Meanwhile my wife thinks there may be air in the line.
Is that possible? Could that be a problem? How do I get rid of it?

Regards,
Captain Dave
BOAT log/Blog

The air would have purged fairly quickly, I'd think.

But you will probably have to go back in... If you can get at it with a long "bottle" brush, turn off the water main and run the brush down through the opened valve to clean any gunk that's been worked loose. You may end up having to replace the valve...

Btw: nice Catalina! Excellent choice!