This noise occurs when the turntable roller rest and cavity bottom are dirty. Frequent cleaning of these parts should eliminate or reduce the noise.

Why is standing time recommended after the cooking operation has been completed?
With microwave cooking, the heat is in the food, not in the oven. Many foods build-up enough internal heat to allow the cooking process to continue, even after the food is removed from the oven. Standing time for joints of meat, large vegetables and cakes is to allow the inside to cook completely, without overcooking the outside.