In the last few months I have noticed that when the hot water is turned on to an infrequently used shower (but not the bath faucet) the pipes groan and rattle. This sound and vibration immediately ceases when the diverter is switched back to the tub or the water is turned off. Cold water does not make any noise.
I have tired a few suggestions that I found on the web such as taking off the shower head to clear air and shutting off the hot water and draining all of the lines in the house. So far nothing has worked (I am so glad that one of those links was to this forum.) I am not very experienced with plumbing and the sound and vibration really concerns me.
The fixtures seem kind of old. Since this has been happening it also seems like the water pressure the the shower has gone down.

Any ideas out there?