Alarming noise in the movement

[praezis]

Hi,
finished overhaul of a WW2 pilot chronograph: Watch ran already for 24 hours on the Cyclotest, then 6 h with active chronograph, now a last check on the timing machine. All looks good, displayed values, trace, scope.

But suddenly there is an unexpected alarming noise coming from the loudspeaker! One time it is exactly on the escapement noise, then a bit later, intermittent, with several 10 seconds between. However the trace stays straight, amplitude value has no decrease at all.
What could be the cause of this annoying noise??

Frank

noise1.mp3

[Chopin]

Kind of sounds like metal hitting metal ?

[oldhippy]

This could be the pinging sound you sometimes get from the hairspring vibrating between the regulator pins.