Source of grinding sound from MBA
In most cases your Mac will cool itself down by passive airflow, but when things get too hot, the fan will spin up to get the excessive heat out.

Chances are that the drop caused a minor bend in the casing.  While you might not see any damage with the naked eye,the MBA is engineered at micron level and there's little room for any "unnatural" dimension changes.

However you might be (arguably) lucky, and the drop might have damaged the fan closure or fan blades.  The fan is fairly easily replacable at Apple Service, the casing will be less replacable (or too expensive to do so).

I think it's time to get your Apple Care plan executed, or to give your insurance agent a call.