From the overhead silhouette seen from a distance, I'm pretty sure that what I have seen on numerous occasions is a Cessna Skymaster and if that is the case these are some of the loudest conventional plane engines I have ever heard. Maybe something to do with the push-pull configuration? :
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They are known for being loud, and on the inside as well. I think the choppy air from the front prop makes the sound waves from the rear propagate in a bad way, or something.