Two days ago I replaced the fill valve, wall shut-off valve, and connecting hose on a second-floor toilet. The house was built in 1993/4 with all copper pipes. Everything seemed great - no more constant filling of the tank, no more dripping at the shut-off valve. Happy.

Yesterday I did the same thing for a first-floor toilet in the same house. Used same make and model of fill valve. Even managed to get the old compression ring off the copper pipe to put the new one on. No more running in the tank, no more dripping at floor. So far, so good.

Then out of the blue this high-pitched whistling or humming sound began, almost like a tuning fork, and grew louder as it resonated. Flushing the toilet made it go away. I ran the faucets to get any remaining air out of the lines, but this noise keeps coming back intermittently in the pipes, and it's driving me nuts.

An internet search for "humming pipes" often blames this on a toilet fill valve needing to be replaced, but I just did that. Twice. The noise only started after I did the second toilet on the first floor.

If it's important, I used this model: HydroClean HC660

Any advice is greatly appreciated. Do I really need to replace the valves again? Is this something that will work itself out over a couple of days? Thanks everyone.