  We include the effect of the frictional force caused by interactions between
cosmic strings and the particles of the background plasma in the computation of
the stochastic gravitational wave background generated by cosmic string loops.
Although our results show that friction leads to a partial suppression of the
emission of gravitational radiation by cosmic string loops, we also find that
loop production is very intense in the early stages of the Kibble regime. We
show that, in many instances, this leads to a prominent signature of friction
in the ultra-high frequency range of the spectrum, in the form of a secondary
peak. The signature of friction is not only sensitive to cosmic string
properties, but also to the initial conditions of the network and its
surroundings. A detection of this signature would then allow us to extract
information about the physics of the early universe that cannot be uncovered
when probing the rest of the stochastic gravitational wave background spectrum.
