I realize this is an old thread, but I'm a new member to this forum and the thread is new to me.
I'm not sure what options are in the car but the E36 has a reminder that you can set that beeps 3 times at the same minute of every hour. Designed to be a reminder for weather or news that are always at a same time (i.e. AM radio)
I didn't think this was an E30 option, but it's a random chance this may be enabled on the computer or clock? The icon on the E36 looks like a period with several lines to the right of it.