yes hello the reason why this nosie is happening is today water heaters come with pre-installed "heat trap nipples" basically they have these small little balls in them.. (that explains the ball bearing rattling noises) there supposed keep the heat in. i have had sevral customers call and complain about this problem i just remove them and install brass nipples on all my water heaters.
so remove the short hot and cold nipples from the tank and look to see if you have little balls or some times rubber dampers/ if so install brass nipples if you have copper pipe / or galvinized if you have steel pipe don't forget unions. good luck
- Sean the plumber