Dryer is ka-thunking like there's a shoe trapped inside...

Dryer is ka-thunking like there's a shoe trapped inside...

When I turn on my dryer there's a ka-thunking sound, constantly, when it is drying. It dries fine. It traps lint properly and no other issues does it have but that… seems to still spin too. But manually turning the drum is a little difficult when it's not turned on. What's the issue?

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