  We report the detection of a short-lived narrow quasi-periodic oscillation
(QPO) at ~88 mHz in an Insight-HXMT observation during the soft state of the
persistent black hole high mass X-ray binary Cygnus X-1. This QPO is
significantly detected in all the three instruments of Insight-HXMT, so in the
broad energy range 1-250 keV. The fractional rms of the QPO does not show
significant variations above 3 keV (~5%) while decreases at lower energy (~2%).
We show that this QPO is different from the type-A, -B, and -C QPOs usually
observed in black hole X-ray binaries. We compare QPOs at similar frequencies
that have been previously detected in other persistent high-mass X-ray binaries
in the soft state; we speculate that such QPOs might relate to some local
inhomogeneity rarely formed in the accretion flow of wind-fed accretion
systems.
