Why does my space heater make a sizzling, popping sound when on highest setting?

I bought a space heater today at a yard sale. I think it is prefilled with oil and not supposed to be refilled. I plugged it in for a while on the highest setting and the heater gets hot but is making a loud popping, sizzling sound which sounds like hot grease popping on a stove, a crackling sound. I am not sure... show more I bought a space heater today at a yard sale. I think it is prefilled with oil and not supposed to be refilled. I plugged it in for a while on the highest setting and the heater gets hot but is making a loud popping, sizzling sound which sounds like hot grease popping on a stove, a crackling sound. I am not sure when the last time was that the heater was used but am wanting to know if this is normal? When on the lowest setting it does not make any noise but only on the higher settings.
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