The best clue is the nature of the sound, but given that you only hear it when riding, and your weight, my best guess -- only a guess on such limited info -- is a hub bearing problem, or possibly a broken axle.
Remove the rear wheel and rotate the axle to feel for roughness. Most likely, opening it up, cleaning and greasing will solve it.
One diagnostic test, if there's a shallow grade handy, is to install the rear wheel reversed (cassette on left), with the chain passing under so it doesn't rub, and hung to the chain stay do it doesn't drag, then sit on the bike and coast down the hill. If you hear the same noise, it's in the hub.
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