Downstairs shower knocks when hot water is on
Ok, so my house has 2 full baths, and we're having problems with the downstairs bathroom.
The shower has a single temperature/flow control knob/lever, a tub spigot with shower diverter, and a drain lever.
If you use hot or rather warm water, as normal, the pipe knocks, shaking the wall, and making a hideous noise that you can hear everywhere else in the house.
You can make the noise go away in three ways:
1) Take a cold shower
2) If you turn down the RATE of the water, so you're not using full pressure, you can take a shower of any temperature without noise
3) If you lower the shower diverter a small amount (so some water goes out the tub spigot in addition to the shower head) you can also take a shower of any temperature without noise.
My first thought is... the tub spigot has something to do with air getting into the pipes. This may have something to do with the fact that when I use that shower, I turn the diverter after the water is on... but the noise exists even if you turn on the water while the spigot is up...
Can anyone help? Will replacing the tub spigot help? I don't have access to the pipes on this bathroom like I do on the upstairs, so I'm hoping I don't need to get into the wall much for this...
[ on a related note, the upstairs bathroom shower leaks from the tub spigot, can that be fixed without replacing the spigot? ]
