[MBAtarga]

yes- yellow item is a transformer. I'd suggest removing the power source and removing the board from the mounting location. Look on the underside at the solder connections over the entire board. It might be there is a joint with a small crack - and the tap vibrates the board enough to make connection. If you find anything suspect, use a solder iron to re-flow the solder at those junctions - re-install board and give it a try.