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The density check is to figure out if you have a double density or high density disk drive. Older BIOSes (early 90s?) seemed to do this more often, later on we just assumed HD.

The distinctive buzz-buzz sound is because the controller is moving the drive head from track 0 to 79, and back again.
On an HD 80-track drive, this'll work. On a DD (40-track) drive, it'll actually only go to track 39 and get stuck there. Then when it tries to go back to 0, it'll hit zero early