When the wind blew from a specific direction and with sufficient velocity it caused the back side of the rain gutter (the side closest to the side of the house) to vibrate. If you look at your gutters you may see a slight gap in this spot.
Well the problem was a quick fix and here's what I did.
I took a wood screw (about 1 1/4 inches in length) and a soft plastic spacer. Then used my cordless drill to affix the back of the gutter to the house, thus preventing further vibration.
The next evening another wind storm blew through Oklahoma City and I stepped outside just to check out my handywork.
I stood under the spot I repaired and the strange noise was gone.
Just for good measure I built a small fire underneath that place and offered the neighbors cat as a sacrifice against future haunts.