A loose rear wheel skewer sounds like it's coming from the bottom bracket when you have a load on it. When I start hearing a creak on a ride I'll pull over and tighten it and the majority of the time that's it. The last time which was a few weeks ago it was the pedal axle threads that just needed to be greased. Just check those and if the sound is still there just look at every part that has metal to metal contact. Many sounds sound like it's in the bottom bracket area when it's not.