If your dryer vent system exceeds the length prescribed by building code it may require a booster fan. A booster fan is designed to activate automatically when your dryer turns on and helps to blow the exhaust air through and out of the dryer vent.
Already have a booster fan? It's going to need to be cleaned and checked for proper operation periodically. Lint can build up on the booster fan blades and cause it to vibrate loudly and can clog up (see photo) the fan to the point where it ceases to function.