  Cross-correlations of the WMAP full sky K, Ka, Q, V, and W band maps with the
1.4 GHz NVSS source count map and the HEAO I A2 2-10 keV full sky X-ray flux
map are used to constrain rms fluctuations due to unresolved microwave sources
in the WMAP frequency range. In the Q band (40.7 GHz), a lower limit, taking
account of only those fluctuations correlated with the 1.4 GHz radio source
counts and X-ray flux, corresponds to an rms Rayleigh-Jeans temperature of ~ 2
microKelvin for a solid angle of one square degree. The correlated fluctuations
at the other bands are consistent with a beta = -2.1 +- 0.4 frequency spectrum.
Using the rms fluctuations of the X-ray flux and radio source counts, and the
cross-correlation of these two quantities as a guide, the above lower limit
leads to a plausible estimate of ~ 5 microKelvin for Q-band rms fluctuations in
one square degree. This value is similar to that implied by the excess, small
angular scale fluctuations observed in the Q band by WMAP, and is consistent
with estimates made by extrapolating low-frquency source counts.
