Yeah, a loud knocking isn't normal for these—they're supposed to run super quiet, around 51 decibels. That noise usually points to a serious lubrication or fuel problem.
First, shut it off immediately. Let it cool. The number one culprit is low or contaminated oil. Check the oil level and condition using the maintenance hatch on the side panel. If it's low, top it up. If it's milky or has debris, you need to drain and replace it completely. Do this when the engine is hot for full drainage.
If that doesn't quiet it down: You could have bad fuel. Old gas (over 30 days) or gas mixed with oil can cause detonation and knocking. Drain the tank and refill with fresh, unleaded 87+ octane fuel. Don't mix oil with the gasoline.
When to call it: If you've checked oil and fuel and the knock persists, stop using it. That could indicate internal engine damage. Call WEN customer service at 800-232-1195.