  We report results from joint NuSTAR, Swift and XMM-Newton observations of the
newly discovered black hole X-ray binary candidate Swift J1658.2-4242 in the
intermediate state. We observe a peculiar event in this source, with its X-ray
flux rapidly decreasing by $\sim$45\% in $\sim$40~s, accompanied by only subtle
changes in the shape of the broadband X-ray spectrum. In addition, we find a
sudden turn-on of a transient QPO with a frequency of $6-7$~Hz around the time
of the flux change, and the total fractional rms amplitude of the power
spectrum increases from $\sim$2\% to $\sim$10\%. X-ray spectral and timing
analyses indicate that the flux decrease is driven by intrinsic changes in the
accretion flow around the black hole, rather than intervening material along
the line of sight. In addition, we do not significantly detect any relativistic
disk reflection component, indicating it is much weaker than previously
observed while the source was in the bright hard state. We propose accretion
disk instabilities triggered at a large disk radius as the origin of the fast
transition in spectral and timing properties, and discuss possible causes of
the unusual properties observed in Swift J1658.2-4242. The prompt flux
variation detected along with the emergence of a QPO makes the event an
interesting case for investigating QPO mechanisms in black hole X-ray binaries.
