I'm sorry , but that's not quite correct. The idea behind "flexing" the wheel is to stress relieve the spokes at the bend (as they enter the hub) by yielding them a bit. The bend area of the spoke has a large amount of residual stress from the manufacturing process, which can lead to premature fatigue failure if not corrected.
The ironic part is that the method you describe doesn't stress the spokes past yield, so it doesn't do anything at all. To properly yield them, you need to grab a pair in your hands and give them a really good squeeze, preferably before the wheel has been tensioned and trued.
I still think the pinging noise is your audible warning that the rim or hub flange is starting to crack. One reason that the noise is disappearing is because the crack is opening up a bit and is that much closer to failure.
John Swanson
www.bikephysics.com
rustynuts said:It might be that when the wheel had a new spoke fitted and was subsequently re-trued, the mechanic didn't "flex" the wheel ('flexing' the wheel is when the wheel is placed on a hard surface and pressure put on the rim sides. The aim of this is to release spoke wind up). If the wheel isn't flexxed the spoke wind up releases itself under hard pedalling, making pinging noises.