Put something soft under the box fan

To stop a box fan from rattling on a windowsill, you need to break the hard plastic-to-hard surface contact with something soft.

The rattling is just vibration. The fan sits on a hard sill, and the plastic feet or frame transmit every little imbalance straight into the window frame. A thin layer of something squishy absorbs that vibration.

Cheapest fix: fold up a dish towel or an old t-shirt and slide it under the fan. Works instantly. If you want something more permanent, a strip of weatherstripping, a few cork coasters, or those little rubber furniture pads do the same thing.

Also worth checking if the fan is wobbling because the blades are dusty. Clean them off—uneven dust buildup can throw off the balance and make the shaking worse.

Five minutes and you’ll stop feeling like you’re sleeping inside a washing machine.