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.
https://www.bing.com/search?q=2006+cadillac+sts+cause+for+a+high-pitched+alarm+sound+that+through+the+stereo+speakers&form=EDGTCT&qs=PF&cvid=4dfa310a813141c8aea141ad00c29e79&refig=3748d854120643c0f624b41d4ec0adf4&cc=US&setlang=en-US&plvar=0
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