Lint Buildup
Buildup of dryer lint and other debris inside the dryer vent can cause all kinds of noises when the dryer is in operation. If airflow is restricted through the dryer vent, passing hot air can whistle and whine on its way through the vent. If your dryer vent has a flapper door and the door is propped open by excessive lint buildup, a similar phenomenon can occur. Regularly clean the dryer vent to prevent this from happening.
Animal Interference
Another common cause of unusual noises in a dryer vent is interference from animals. Birds can nest inside a dryer vent housing, or mice and other small animals can crawl inside a vent that is propped open by excessive lint buildup. This can cause rustling, scratching, thumping and many other noises as the animals move in and out of the dryer vent. Remove any animals from the inside of the dryer vent --- contact an exterminator or an animal control service if sanitation is an issue --- and again, clear any lint buildup and other obstructions that may prop a dryer vent door open and give animals a path inside the dryer vent.
Dryer Malfunctions
Certain internal malfunctions on the dryer, such as a stripped-out blower wheel, worn-out bearings or a malfunctioning drum roller or idler pulley, can cause all sort of unusual sounds to emanate from your dryer. While the sounds may not occur in the dryer vent itself, many of the components of the dryer either come in contact with the dryer vent or are used to force air out through the dryer vent, so sounds can reverberate through the vent and give the illusion that they are occurring inside the vent. Troubleshoot the aforementioned components of the dryer or have a professional inspect it to see if this may be the case.
Other Possible Causes
While dryer vent noises may seem only a minor annoyance, they can be an indication of larger problems if left unchecked. If an animal is living inside your dryer vent, for instance, heated air passing through the vent can burn it to death; if left inside the vent, the animal can rot and raise all sorts of health and sanitation concerns. If a dryer part is malfunctioning and the resulting sounds are reverberating through the dryer vent, make sure to resolve the problems right away to avoid a catastrophic failure of your dryer that will be much more costly to repair.