Hi,
I agree it sounds like the spokes but the only other thing I can thing is the rear dropouts being misaligned if you have had a crash or drop and or (and you don't mention whether you are using quick release) the quick release tension is not right. Even if it is but the rear drops are mis aligned it will enable the wheel to move slightly under force. Oiling the spokes would run down and lubricate this joint eliminating the creak. All that said if the spoke tension isn't right then true or not it wasn't done correctly in the first place.
Hope this helps.