Wood will creak due to the moisture content in it. Depending on what part of the country you are from, in the summer months the wood will expand if an air conditioning unit it not removing latient heat (moisture in the air) also know as humidity. When it starts to get colder out the humidity in the house drops and the moisture content of the wood drops with it, the wood shrinks. That could be why you hear the stand creaking. I would put a shim under it any way just to be safe.
