Only the hot water turned on very lightly do the pipes rattle. We live in an old house and none of the pluming is braced to the floor joises above. If the hot water is turned on fully there is no rattle. Could this just be a venting problem, with some air in the line?

There is no vent in a water line, there is no air in a water line. The rattling is caused by a loose washer in a valve somewhere, most likely the one you are fiddling with.

Yeah, as Harold said, most likely a loose washer at the shutoff you are working with causing the issue.

What happens if you open the shutoff all the way? Most likely, it will shudder for a few seconds while opening, but will be OK when fully opened. If not OK when fully opened and the shutoff vibrates when water is used you can change out the washer in the shutoff or just replace the entire shutoff.

Post back with your thoughts, OK?

Mark