cooling system makes rumbling sound....

[ellisjay2]

After driving and parking I notice I hear my coolant rumbling/ making noiling sound, Ive checked under the hood when it does if for a few seconds or so, theres no coolant leak, engine doesnt run hot, recently replaced water pump, sounds to me like its some kind of vapor lock issue or could it be air in the system?? Recentley did cooling system flush with vinegar to remove rust debris from left over non distilled water staying in system. Heat blows heat as it should... im guessing its something minor, just would like some input on what could be causing it to boil/rumble.