  We perform multiwavelength time-distance analysis of a fan loop system
anchored in an isolated sunspot region AR 12553. The active region was observed
by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory. We
measure the phase speeds of the propagating intensity disturbances by employing
cross-correlation analysis, as well as by obtaining the slopes in xt-plots. We
obtain original as well as de-trended light curves at different heights of the
time-distance maps and characterize them by performing Fouri\'er and Wavelet
analysis, respectively. The time-distance maps reveal clear propagation of
intensity oscillations in all the coronal EUV channels except AIA 94 and
335~{\AA}. We determine the nature of the intensity disturbances as slow
magnetoacoustic waves by measuring their phase speeds. The time-distance maps,
as well as the de-trended light curves, show an increase and decrease in the
amplitude of propagating 3-min oscillations over time. The amplitude variations
appear most prominent in AIA 171~{\AA}, though other EUV channels also show
such signatures. Fouri\'er power spectrum yield the presence of significant
powers with several nearby frequencies between 2--3 minutes (5--8 mHz), along
with many other smaller peaks between 2--4 minutes. Wavelet analysis shows an
increase and decrease of oscillating power around 3-min simultaneous to the
amplitude variations. We obtain the modulation period to be in the range of
20--30 minutes. Our results provide the viability of occurrence of phenomenon
like `Beat' among the nearby frequencies giving rise to the observed amplitude
modulation. However, we cannot, at this stage, rule out the possibility that
the modulation may be driven by variability of an underlying unknown source.
