Mine did this off and on for the whole winter. I think that when the engines get some miles on them, they will have some carbon deposits. When they sit, the pistions can come into contact with outside air if that exhaust valve is open. The carbon can then release from the piston because of drying out and then it will knock for a minute or two...
JUST MY THEORY
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 11 YRS