  We have discovered a 7.06+-0.08 Hz quasi-periodic oscillation (QPO) in the
X-ray flux of the low-luminosity low-mass X-ray binary (LMXB) and atoll source
4U 1820-30. This QPO was only observable at the highest observed mass accretion
rate, when the source was in the uppermost part of the banana branch, at a 2-25
keV luminosity of 5.4x10^37 erg/s (for a distance of 6.4 kpc). The QPO had a
FWHM of only 0.5+-0.2 Hz during small time intervals (32-s of data), and showed
erratic shifts in the centroid frequency between 5.5 and 8 Hz. The rms
amplitude over the energy range 2-60 keV was 5.6%+-0.2%. The amplitude
increased with photon energy from 3.7%+-0.5% between 2.8 and 5.3 keV to
7.3%+-0.6% between 6.8 and 9.3 keV, above which it remained approximately
constant at ~7%. The time lag of the QPO between 2.8-6.8 and 6.8-18.2 keV was
consistent with being zero (-1.2+-3.4 ms).
  The properties of the QPO (i.e., its frequency and its presence only at the
highest observed mass accretion rate) are similar to those of the 5-20 Hz QPO
observed in the highest luminosity LMXBs (the Z sources) when they are
accreting near the Eddington mass accretion limit. If this is indeed the same
phenomenon, then models explaining the 5-20 Hz QPO in the Z sources, which
require the near-Eddington accretion rates, will not hold. Assuming isotropic
emission, the 2-25 keV luminosity of 4U 1820-30 at the time of the 7 Hz QPOs is
at maximum only 40% (for a companion star with cosmic abundances), but most
likely ~20% (for a helium companion star) of the Eddington accretion limit.
