  We present the long term evolution of the timing properties of the black hole
candidate Cygnus X-1 in the 0.002-128 Hz frequency range as monitored from 1998
to 2001 with the RXTE. The hard state power spectral density (PSD) is well
modeled as the sum of four Lorentzians, which describe distinct broad noise
components. Before 1998 July, Cyg X-1 was in a "quiet" hard state characterized
primarily by the first three of these broad Lorentzians. Around 1998 May, this
behavior changed: the total fractional rms amplitude decreased, the peak
frequencies of the Lorentzians increased, the average time lag slightly
increased, and the X-ray spectrum softened. The change in the timing parameters
is mainly due to a strong decrease in the amplitude of the third Lorentzian.
Since then, an unusually large number of X-ray flares have been observed.
During these "failed state transitions", the X-ray power spectrum changes to
that of the intermediate state. Modeling this PSD with the four Lorentzians, we
find that the first Lorentzian component is suppressed relative to the second
and third Lorentzian. Also the frequency-dependent time lags increase
significantly. We confirm the interpretation as failed state transitions with
observations from the 2001 Jan. and 2001 Oct. soft states. Such behavior
suggests that some or all of the Lorentzian components are associated with the
accretion disk corona. We discuss the physical interpretation of our results.
