  We establish a probabilistic representation for a wide class of linear
deterministic p.d.e.s with potential term, including the wave equation in
spatial dimensions 1 to 3. Our representation applies to the heat equation,
where it is related to the classical Feynman-Kac formula, as well as to the
telegraph and beam equations. If the potential is a (random) spatially
homogeneous Gaussian noise, then this formula leads to an expression for the
moments of the solution.
