| If it knocks, let it in. OK, seriously.
Your knock is called "water hammer". When the flow of water is abruptly stopped in a pipe, it tends to move the pipes and the pipes generally hit something. Cold generally uses a larger pipe and therefore is more susceptable to hammer. Typically there is pipe a few feet long going upwards to no where at the meter. This pipe is filled with a column of air that cushions the hammer a bit. There are specialized devices for this too. Those little pipes going nowhere are occaisionally placed at washers where the solenoid valve can abruptly close.
Enough of that.
Remedies:
1. Drain entire house and refill which will file the air columns again. Your going to have to drain the column at the meter through the thumbnuts on the valve (hopefully they are there).
2. Secure the loose pipes. Use commercial brackets: The flange+all thread+pipe clamp. Parts purchased separately.
3. Could there be a possible problem with the valve itself? Worn? Play? Packing not tight?
4. Install local water hammer preventer.
I'm not sure, in what order I would try. You should be able to identify 2 and 3 easily. 1-you just have to do. | 
Linear Mode
