We report measurements of the low frequency current fluctuations of a tunnel junction placed at very low temperature biased by a time-dependent voltage V(t)=V(1+cos 2πν t). We observe that the excess noise generated by the ac excitation exhibits quantum oscillations as a function of the dc bias, with a period given by hν/e with e the charge of a single electron. This is a direct consequence of the quantum nature of electricity in a normal conductor.