My friend told me that UA does almost all approaches by hand rather than doing AP-coupled-approaches (obviously not including CAT II and CAT III). So it could be anumber of things including AGPWS, landing gear set/locked/confirmed reminder, radar altimeter passing through ? AGL feet. Heck, even the C182 my friend flew (I was safety pilot) had a whoop when passing through 500 feet. Not sure what that was linked to though and forgot to ask him. Maybe it was the FA call button requesting another root beer.