Its most likely either be the brass fitting soldered to the copper pipe where the nipple is screwed into or the nipple needs to be tightened. My bet is that the brass fitting is cracked, they tend to do that often. The copper pipes and fittings flex a lot more before they break, the brass will just crack. Either way its probably easier to go in from the wall you are talking about. If you have a tub surround, it will be a royal pain, and will be alot harder if you have a tile surround. Use a utility knife and cut out a square gradually. You will need to screw thin plywood behind the sheetrock that is there so you have something to secure the piece you cut out onto when you go to patch it up. When it is cut out just run the water and check for the leak, if it is hard to find the exact location, use soapy water or leak detector on all the possible locations and look for the bubbles.
Technically this is correct; but the magnetic field would be rather very weak, and during the banging the bar has to be kept in good alignment to the earths magnetic field because that is what is creating the magnet. Vibration and heating in fact damage a magnet; in your example however you do have a method to reverse the process and make one.