Jan,
Turbines and their associated dynamics sometimes seem unpredictable. More than likely differences in temperature, spool-up times etc could trigger this.
I have seen something similar in the bypass system of the L-39 from Germany.
I am currently very involved in a CSIR/ Stellenbosch University effort to breed new young Msc and PHd aeromotive engineers.
We are developing new microturbine tech as a learning and research tool.
More than likely they are going to end up in the USA or Europe.
Too late for me to escape.
Cheers
Andre