  The measurement of azimuthal modulations in hadron leptoproduction on
unpolarized nucleons allows to get information on the intrinsic transverse
momentum $\langle k_T^2 \rangle$ of quarks in a nucleon through both the Cahn
effect and the Boer-Mulders function. We have compared the azimuthal
asymmetries in the cross section of $160\, \rm{GeV}/c$ muons scattered off an
unpolarised deuteron target as measured by COMPASS with a Monte Carlo program,
based on the ${}^3P_0$ model of quark anti-quark pair production at string
breaking, which accounts for the Cahn effect. Large differences have been
observed between data and Monte Carlo, in particular at large values of the
fraction of the longitudinal momentum of the fragmenting quark carried by the
produced hadron. We found out that most of these differences are due to pions
from exclusive vector mesons contaminating the SIDIS sample, which also exhibit
large azimuthal modulations. Using the measurements of the exclusive reaction
$\mu N\rightarrow \mu' \, \rho\, N$ we had done in 2006, we can reproduce
reasonably well the observed differences. Subtracting the contribution of
hadrons produced in the decay of exclusive vector mesons from the SIDIS
unpolarised azimuthal asymmetries is therefore a prerequisite condition for
extracting $\langle k_T^2 \rangle$ and the Boer-Mulders function.
