  We analyze ~600 ks data from the X-ray observatory Rossi X-ray Timing
Explorer of the neutron star low mass X-ray binary and atoll source 4U 1636-53.
These observations span almost three years, from April 1996 to February 1999.
The color-color and hardness-intensity diagrams show significant secular shifts
of the atoll track, similar to what is observed for some Z sources. These are
most evident in the hardness-intensity diagram, where shifts in intensity up to
20% are observed. We find that the intensity shifts in the hardness-intensity
diagram are responsible for the parallel tracks observed in the kilohertz quasi
periodic oscillations frequency vs. intensity diagram. While the parallel
tracks in the frequency vs. color plane partially overlap, systematic long term
shifts are still evident. We also study the broad band power spectra of 4U
1636-53 as a function of the source position Sa along the track in the
color-color diagram. These power spectra are all characteristic of the lower
and upper banana state in atoll sources, showing very low frequency noise, band
limited noise, and sometimes one or two kHz QPOs. We find that the very low
frequency noise in some intervals is not well described by a power law because
of an excess of power between 20 and 30 mHz, which can be fitted by a
Lorentzian. Also, the characteristic frequency of the band limited noise shows
a trend to decrease with increasing Sa at high Sa values.
