I've never used sour cream but, that sounds loverly. I usually use a roux(2 tbsp butter, 2tbsp butter) Sometimes I put (minced, fine diced)onions in the butter and sweat them(pinch of salt, couple minutes) first, then add flour(making a roux).
Cook one minute to get the "raw" taste out of the flour(cooking longer can increase nuttiness, if that's your goal).
Add 2 cups milk, stirring frequently until it thickens(you have a bechamel now).
Add 2 cups of your favourite cheese, stir until it melts.(you have classic cheese sauce, substitute parm reggiano and a pinch of nutmeg, you'd have alfredo).
Flavour as you normally would(whatever your mac and cheese recipe calls for) and pour over elbows.(or whatever pasta). sprinkle with some extra cheese if you like, or breadcrumbs, or bacon, or scallions(get creative). Bake as you normally would.(375 30 minutes)
Sorry I'm not
@Sondi but, hope this helps.