somehow...Did it just start happening all of a sudden....? did it happen after you replaced a faucet or something else like a hot water heater ? is it hot water or cold water side only or both? all I can suggest is that you just let it run for about five minutes and let the air flush out of that particular line. not sure if this is caused by a problem with your water hammer dampers in the attic ...they are usually just vertical standpipes on the water lines going to your faucets to prevent the water hammer effert as you turn off the water ..and prevents the loud thuds or clunk noise what water is turned off at the faucets. they hold a column of air in the pipes to cushion the sudden flow of water being turned off. LJ