Big rattling noise in the wall! Seems to be OK if I turn on the faucet. This hasn't happened in the 20 years we've been here, but has happened 4 times in last month. Sometimes happens with a downstairs flush too, or when we do both close to the same time. Could this be air in the lines? How do we get rid of it and keep it from happening again?

It sounds like one of your angle valve has a loose washer, close the angle valves under the sink 1/2 a turn clock wise and watch if it is fixed, if noise still exist then do the same to the toilet angle valve under the thank, if no noise at any codition above then you need to replace the washer at the end of the stem, good luck.

John