The effect of dilute impurity spins on the low-energy magnon spectrum of an impure Heisenberg ferromagnetic crystal lattice is studied with particular attention paid to the case in which each impurity spin is weakly (and ferromagnetically) exchange-coupled with its surrounding host spins. From a solution for one-impurity problem simple analytical expressions for the energy, the width and the localization of Γ1-type virtual bound or resonant magnon mode is obtained using the effective-mass approximation for the evaluation of magnon Green's functions. It is shown that for such an impurity-host system the Γ1-type magnon impurity mode is very likely to occur near the bottom of the magnon energy band. A brief discussion is given of the magnon specific heat at low temperatures.