We propose a self-checking scheme for the on-line testing of power supply noise exceeding a tolerance bound to be chosen accordingly to system?s constraints. Upon the occurrence of such a noise, our scheme provides an output error message, which can be exploited for diagnosis purposes or to recover from the detected noise (thus guaranteeing the system?s correct operation). As far as we are concerned, no on-line testing scheme for power supply noise has been proposed up to now. Our scheme negligibly impacts system?s performance, features self-checking ability with respect to a wide set of possible internal faults and keeps on revealing on-line the occurrence of power supply noise, despite the possible presence of noise affecting also ground.