Narrowband electrostatic noise (NEN) is frequently observed upstream of the slow shocks in the distant tail. We suggest that NEN represents a non-standard plasma wave mode which results from a hole in the low energy electron velocity distribution. A very simple model which plausibly indicates how a hole might be formed by the interaction of the electrons with the slow shock is presented. The hole mode is destabilized by a positive slope in the parallel electron distribution and/or by a weak of energetic ions which escape upstream from the slow shock. ¿ American Geophysical Union 1989