Why is a Lockheed MC-130J Commando II creating such a loud droning sound?
There are two factors reducing the volume of a sound when travelling through the air:


The pressure wave expands as the surface of a sphere, which will reduce pressure as a function of distance $r$:

$$ p(r) \sim \frac{1}{r^2} $$

Since human hearing is logarithmic, we typically use Decibels (dB) as a unit for volume, rather than pressure directly:

$$ L_p = 20 \log_{10} \left( \frac{p_\text{RMS}}{p_\text{ref}} \right) $$

This means that an aircraft that is twice as far away will (at maximum) have a sound pressure of one quarter left, which corresponds to $ 20 \log_{10}(1/4) \approx -12 \, \mathrm{dB} $. An aircraft at 27,000ft will only be $ 20 \log_{10}(1/27^2) \approx -57 \, \mathrm{dB} $ less loud compared to an aircraft on final approach at 1000ft. For a large and loud turboprop, this is easily enough to hear it, considering the engines can reach up to 140 dB and aircraft on approach are typically up 80 dB loud:


The absorption of sound waves in air is frequency dependent. This physics.stackexchange answer explains the details. The following plot shows the absorption coefficient as a function of frequency for different humidities:


(image from Wikimedia)


Since turboprops are producing more lower frequency sounds with their propellers compared to turbofans, they can be heard further (they also typically fly lower).
This also matches the "droning sound" you describe. The higher frequency noise has been absorbed by the atmosphere.