Well, are you just changing the lead or the whole power supply in these tests. The effect is a well known electrostatic effect, Indeed, some suggest that this sensitivity in our hands and nerves thereof are why dowsing works for many people, but I digress, I can for instance, go and stand under some power lines, and touch the metal parts of my blind cane and feel the tingle.
Hardly surprising that its so powerful when much closer to a lower voltage then. Its not really dangerous and some two core lead appliances like hi fis suffer the same way. Often ist can be made worse by some capacitors in the power supply that are there to help suppress nasty things in from or out to the mains, but if you are just changing the low voltage cable, then this seem not to be logical. I know many phone chargers if you touch the metal bit of their phone plugs give the same effect as well. Switch mode supplies seem to be prone to this for some reason, which is peculiar because the frequencies they are generating internally are much higher than 50 hz.
Brian