I have a two story house and noticed a water stain on the wall at the ceiling on the first floor in my laundry room which is below the water heater. I found no water upstairs around the water heater. I cut a hole in the wall down stairs and found the hot & cold water lines from the water heater. The hot line had water leaking down it but I can not see from where because the copper pipe goes into a 2x4 header one on top and one below of what I think are two 2x12 floor joist. Is it likely the copper pipe is leaking? Can not tell if it has a joint in the part that's hidden in the joist. I turned off the hot water line at the water heater and the leaking stopped. So its got to be the hot water line right? So I turned it back on just to make sure and now no leak. What the heck? This makes no sense to me! Also I have now lost some water pressure in the shower upstairs

There are a lot of homes that used the wrong gauge pipe in construction and now the pipe is wearing out. You should be able to see where the pipe connects to the water heater and it either goes into a wall and down the wall or straight through the floor. Regardless of what it is doing now, It should be looked into further to make sure that it is not going to make a huge mess. It might be sweating pipes but I doubt it. The pressure issue is troubling. Did you turn it back on all the way? Have you performed a flush on the water heater to get all the sediment out?

It is turned on all the way and I have not flushed the water heater. I should have stated that the water heater is 21 years old(gas). But I do not see any water leakage upstairs around the water heater only after the line comes straight down out of the floor joist. Not above the joist.