On Tue, 06 Nov 2007 14:07:25 -0000, Joe wrote:

I thought I had fixed this problem last winter but now it's back
again. When water is turned off upstairs the cold water pipe rattles
against the drywall. It started happening the same time last year. It
seems that when the outside temp falls down to the 40s and 30s is the
only time it happens. Does the ground being colder have something to
do with the water being heavier?

Everything has a natural vibration frequency. Changing physical
properties of an item will change this frequency. The typical "fix"
is to add additional clamps or bracing close to the source, or add a
hammer arrester close to the tap. And, yes it is possible the
temperature change can cause a vibration. Cooler water is slightly
heavier and moves slower than water at higher temperatures.