An upstairs bathroom exhust fan piping system has all of a sudden started making a new noise. When in use, the piping system that runs across the house and out the side of the house has begun to flip open and closed, making a great deal of clicking noise. Why would it start doing that?

My first answer is to make sure that the piping from the exhaust fan through to the vent is clean. Debris in the vent line can cause poor air flow.

Similarly, turn the power off to the fan, and take it out and look at the fan blades or impeller blades. If these are clogged, then you will get the same type of air flow problems.