noise under the floorboard

I have a static drop and ever since then ive had a noise and vibration the seem to be coming from underneath the floorboard under the floormat on the drivers side. It only happens when I am going about 55-60 mph and when the engine is at 3000 rpm it also only happens when the engine is under a load like when I accelerate at 55-60mph. I am curious on what is going on it only happens at highway speeds and when I accelerate if it is at a constant speed it makes no noise only when I acclelerate or decellerate. Any suggestions woudl be appreciated.