whistling mac plus

i'm putting together a mac+ for someone and while testing it, i noticed it started clicking inside. then later, the machine started making a high pitched whistling noise, almost kinda like what you hear when you're close to the back of a tv while it's on. the video worked fine however. i presume it's a power supply problem, but can anyone say what component is failing here? smacking the left side of the case would make the whistling quit, although the clicking continued. i fear it's probably that infamous cold solder joint issue. i'm looking at a spare mac power supply board; does anyone know any exact locations of common solder joint failures? finally, if i do touch up some contacts with a soldering iron, MUST the high voltage lead be discharged? i'm hoping i can just remove the protective cardboard cover, reheat the solder joints and not even go near the HT lead or tube at all. david