  We re-analyzed two Rossi X-Ray Timing Explorer archival observations of the
atoll source 4U 1820-30 in order to investigate the detailed time-frequency
properties of the source during the intervals when a ~7 Hz QPO was detected by
Wijnands et al. (1999, ApJ, 512, L39). We find that in both observations, in
addition to a QPO signal lasting a couple of minutes as previously reported,
there is a much narrower transient oscillation with a life time of only a few
seconds. Within this time, the oscillation is consistent with being coherent.
Its integrated fractional rms is around 10% and its frequency 7.3 Hz and 5.7 Hz
in the two observations. We discuss the possible association of this QPO with
other oscillations known both in Neutron-Star and Black-Hole systems,
concentrating on the similarities with the narrow 5-7 Hz oscillations observed
at high flux in Black-Hole Candidates.
