I've just swapped over the wheel from my singlespeed turbo trainer bike. I had in mind at the time i put that together that by using another 19 sprocket in the same position on the freehub, I had a backup wheel for the singlespeed road bike.I was about to suggest that you carefully inspect the welds and joins at each spot on the bike, so dropouts, bridges, tube intersections. I had a constant creaking on my Felt, turned out to be the top tube/seat tube weld.
I had a turbo tyre on the second wheel so I've put an old road tyre on instead. Once the temperatures have dropped off a bit I'll go for a ride to test the bike with this wheel.
I will take a look inside the original freehub anyway. It wouldn't hurt to clean it and give it a light lubing.