  ULX-7, in the northern spiral arm of M51, demonstrates unusual behaviour for
an ultraluminous X-ray source, with a hard X-ray spectrum but very high
short-term variability. This suggests that it is not in a typical ultraluminous
state. We analyse the source using archival data from XMM-Newton, Chandra and
NuSTAR, and by examining optical and radio data from HST and VLA. Our X-ray
spectral analysis shows that the source has a hard power-law spectral shape
with a photon index Gamma~1.5, which persists despite the source's X-ray
luminosity varying by over an order of magnitude. The power spectrum of the
source features a break at 6.5^{+0.5}_{-1.1}x10^-3 Hz, from a low-frequency
spectral index of alpha_1=-0.1^{+0.5}_{-0.2} to a high-frequency spectral index
of alpha_2=0.65^{+0.05}_{-0.14}, making it analogous to the low-frequency break
found in the power spectra of low/hard state black holes (BHs). We can take a
lower frequency limit for a corresponding high-frequency break to calculate a
BH mass upper limit of 1.6x10^3 solar masses. Using the X-ray/radio fundamental
plane we calculate another upper limit to the BH mass of 3.5x10^4 solar masses
for a BH in the low/hard state. The hard spectrum, high rms variability and
mass limits are consistent with ULX-7 being an intermediate-mass BH; however we
cannot exclude other interpretations of this source's interesting behaviour,
most notably a neutron star with an extreme accretion rate.
