Brian Gaff (Sofa) explained on 18/08/2020 :
What does it sound like is it a modulated signal or a frequency shift keyed
thing.

It is carrier on/off keyed once per second, with a fast on/off at the
epoch. Length of on versus transmits time and date data, at one bit per
second. The 'fast' on/off is called the croak and transmits the fast
data burst - all data sent within the epoch second.