Hello Marc,
the provider is carrying your E3 (that is PDH) inside an SDH infrastructure.
In this case the provider can say it is not providing a clock directly, but SDH has its own clocks that uses to sample the PDH E3 link.
You can try by issuing the following:
sh int ser x/y
if inside the input error counters you see frame errors this can point to a clocking problem.
If this is the case the two clocks are free running.
From time to time deviations become so big that SDH is not able to sample the PDH link and the line has to be reset and this is what happens with the loop test.
If on one side you set the clock internal you provide a reference to the other side and they are not free running anymore.
Hope to help
Giuseppe