PPRuNe Forums - Beep sound on startup of the A320

Hi guys! Me and an engineer have been both breaking our heads over this, and no matter what we do we cannot figure this out. On certain A320 family aircraft, when the APU initializes/external power generator is turned on, there are a series of noises. First of course is the relay. Then there is the TR unit/alternator which makes a high pitched whine. But then there are a few differences which I do not understand.

In about half the videos of A320 startups, there is a high pitched single or double beep (videos attached below). I got told by some that it is the cockpit door lock/video system, others that it is the electrical system initializing, but nobody has a concrete answer to this.

Another thing, there are CIDS chimes when the aircraft gets powered. Strangely on some aircraft the CIDS chimes are slow and continuous once the battery is started. On others, once ext/apu power is on, there are two quick chimes and nothing when batteries are on

If someone knows for sure what this is, please do tell me, because me and the engineer would really appreciate knowing this, there are a few variations of the sounds I have heard. I cannot attach videos or files, but one video on youtube called "Airbus A320, Arranque de APU con Baterías." has a single beep double chime," Airbus A320 Powerup" has a double beep then continuous chime, and lastly "Piloting Airbus A320 out of Dublin + Power Up 1st Flight of the Day" has double beep, double chime.