I have received quite a few messages regarding how to "loop" drones/ambiences/soundscapes from my SFX libraries. All drones/ambiences/soundscapes etc from my libraries are both faded in and out, therefore you cannot loop it, at least not in a literal way.
However, if you want to extend the length of drones/ambiences/soundscapes etc the best way to go about it is by crossfading two /or more/ audio clips. Now, there are few types of crossfades - the most popular being "Equal Gain ><", however, if you want to keep loudness at the same level I'd suggest using "Equal Power )(" crossfade. Hope it helps!