It seems to play the beep until it loads a boot sector. If you boot your pcd from floppies, does it last longer?
In the video, I boot from floppy. The beep seems to have the same length whether I boot from Floppy or HD.
In this short audio file you can hear me hitting the power switch, this produces almost instantly a short beep, then the HD spins up, next is the "test end" beep, after DOS has loaded from the HD I type "klickon" and then press a key a few times before I let it autorepeat by keeping the key pressed.https://dl.dropboxusercontent.com/u/55419307/Siemens%20PC-D/pcd%20start%20klickon.mp3
The only reference for 66Hz I could find at first glance is screen refresh.