There are a couple of causes reported online for this. One is that the OnStar unit is malfunctioning and it needs to be fixed. The other is a random beep tone that lasts from a few seconds to serveral minutes, espeically when starting the car. It has been traced to the Bose amplifer (driver side trunk) where a new one from the dealer costs $700, but you maybe able to find it cheaper on eBay or something like that.
I picked this up from a BING search, so take it for what is worth. This is from different reports online. Best of luck.