I've seen lots of posts that speak of a single toilet whining, but one day, a few weeks ago, all of our toilets on the middle and upper floor (3 toilets) started whining when the tank is refilling.
If I turn on the cold water faucet next to the toilet (or the shower) the whining immediately stops!
So I thought this may be a water pressure issue. .. but our pressure doesn't seem to be very high. Anyone have any ideas?