What is this thing inline with my dryer vent?
Acme Miami Model 9460 Dryer Vent Booster

If the vent has a long run to the external opening (up to 100 "effective" feet), this unit has a fan and a pressure sensor to automatically turn on when it senses dryer outflow to help push the exhaust air out.

When properly adjusted per instructions, the fan motor should come on when the dryer is in operation.