It can be stray electrical voltage causing a current through body capacitance but FEEL mechanical. I think many of us have experienced this (!). Touching and pressing down may cause it to stop due to a lower resistance to body capacitance, but lightly touching while moving a finger across can feel like a 100/120Hz mechanical vibration, but is actually caused by a small electrical current.
The answer to original question is, I reckon, either "magnetostriction" or "Lorentz forces" or both. The repeated change of magnetization of the transformer core causes it to mechanically deform a tiny amount (really small), and the Lorentz forces between current-carrying wires in the windings can cause them to move if not well glued together with lacquer. If the mechanical assembly happens to have a resonant frequency near twice the mains frequency it can amplify either of these effects significantly.
Only 2-prong switch-mode supplies have enough leakage current through inter-winding capacitance to produce the "tickling effect", and they tend to produce a whole lot of switching noise interference too due to the lack of grounding.
Only 2-prong switch-mode supplies have enough leakage current through inter-winding capacitance to produce the "tickling effect", and they tend to produce a whole lot of switching noise interference too due to the lack of grounding.
