I had an occurence once with a passenger flight.
Reduced collective and it stopped, until I raised it again.
Nursed the bird back to base with the occasional surge again - looking for autorotative reachable spots all the way... calming down the passengers ;-)
When they striped the engine they found, that parts from the compressor housing, there where the compressorwheels spin, went through the engine.
After seeing the amount, which went through, an outside landing would have been the better decison....
If you donīt have knowledge of i.e. salty environment causing an engine surge, better treat it as an engine failure on its way....