2012 M6, 143K miles on 2.5. Whenever we start it up cold, there is a gurgling sound like coolant pumping but goes away in 2-3 seconds. Ive topped the coolant at rad cap off, topped reservoir off and ran until warm and watched overflow tank push out my over fill and it seems to settle at Full HOT when I heat cycle it the next morning. It did blow the upper hose and almost got to HOT 2 months ago but son said he pulled it over before it got past the upper heat range. Not sure what happened after it sat very warm with no coolant circulating, probably climbed even higher but it was not running. He waited 30 minutes and reattached the hose and added coolant from the jug, started normally and settled right in middle of range again with no further issues. Could the freon be what Im hearing? I have not checked the freon pressure but I did notice the AC pump cycling at idle about every 20 seconds. 80F here when I did that test. Bugs me more than him.

 

well many platforms and model in those platforms can make those sounds your described.
But based on your replacing the upper hose....
Using ramps like Rhino Ramps after the engine has completely warmed up and even better if you have driven it for at least 30 minutes park up on the ramps and then let it remain on the ramps overnight. The next day start the engine and see if you noise goes away. Also don't forget to recheck the coolant level after this service.

If you don't have ramps a driveway of not less than an 8+ degree incline should work as well.