  By direct measurements of the gas temperature, the Atacama Large
Millimeter/sub-millimeter Array (ALMA) has yielded a new diagnostic tool to
study the solar chromosphere. Here we present an overview of the
brightness-temperature fluctuations from several high-quality and
high-temporal-resolution (i.e., 1 and 2 sec cadence) time series of images
obtained during the first two years of solar observations with ALMA, in Band 3
and Band 6, centred at around 3 mm (100 GHz) and 1.25 mm (239 GHz),
respectively. The various datasets represent solar regions with different
levels of magnetic flux. We perform Fast Fourier and Lomb-Scargle transforms to
measure both the spatial structuring of dominant frequencies and the average
global frequency distributions of the oscillations (i.e., averaged over the
entire field of view). We find that the observed frequencies significantly vary
from one dataset to another, which is discussed in terms of the solar regions
captured by the observations (i.e., linked to their underlying magnetic
topology). While the presence of enhanced power within the frequency range 3-5
mHz is found for the most magnetically quiescent datasets, lower frequencies
dominate when there is significant influence from strong underlying magnetic
field concentrations (present inside and/or in the immediate vicinity of the
observed field of view). We discuss here a number of reasons which could
possibly contribute to the power suppression at around 5.5 mHz in the ALMA
observations. However, it remains unclear how other chromospheric diagnostics
(with an exception of Halpha line-core intensity) are unaffected by similar
effects, i.e., they show very pronounced 3-min oscillations dominating the
dynamics of the chromosphere, whereas only a very small fraction of all the
pixels in the ten ALMA data sets analysed here show peak power near 5.5 mHz.
