The longer you have been using your dryer vent, the greater the chance that you will develop a problem with the venting system. Older dryers are not likely to be as efficient as newer versions, and they may also be suffering from overuse. If your dryer vent starts making strange noises, you will have to perform some quick maintenance of your vent to prevent the situation from getting worse.
Unless the problem is particularly serious, you should not need to call in the professionals for something that you should be able to do yourself. Assuming that you have some basic home improvement skills, you can unscrew a bolt, for example, and then be able to diagnose the noises coming from your dryer vent.
Banging or Rattling
If you are finding that your vent or the vent pipes start rattling and gurgling every time you start up the dryer, you may have one of a number of problems. The most serious problem is the accumulation of lint along the sides of the pipes, and at the end. Lint in the vent is the prime cause of vent fires. Try and get rid of the noises by cleaning away lint. Another problem which might cause rattling is a loose screw.
When you are not using the dryer, walk along the pipe, rattling each one in turn. You may find that one of the bracket screws, which hold the pipe onto the wall, has worked itself loose. Fix this by simply screwing it back into place. Another problem with the vent system might be that some debris is in the pipe. Surprisingly small amounts of sticks, stones and similar items can make loud noises in vents, and while it is easy for them to be carried by the hot air, they often don't make it as far as the vent. Clear away these pieces of debris and you should be free of the noise.
A peculiar whistling noise from the pipework when the dryer is being used may be caused by air escaping through a hole between joints. This can happen quite suddenly and is caused by the pipes expanding and contracting through humidity, and then not settling back in exactly the same position. Try and fit the pipes back together and add a layer of caulk around the joint.
The sound of dripping water coming from your vent is most likely exactly that. Vents can accumulate water vapor when the dry air is carried into the pipe work, and once the air is far enough away from the dryer, it cools, allowing the water to descend to the bottom of the pipes. This can leak through the vent or may be caught somewhere within the system. You should examine your pipes for signs of leaks and look through the hood at the exit for water stains, or a pool of moisture.