My wife drives a 2000 Volvo S40, and we recently drove it from North Carolina to Canada. As we started out on our trip, about 30 seconds after starting the car, something started to make a clicking sound in the dashboard on the passenger side, right underneath the right speaker.
The click occurs about 4-5 times per second. The sound stops after about 20 clicks, and doesn’t occur again until the car is stopped and restarted.
After some investigation, I’ve come to the conclusion that this clicking sound is caused by something in the air conditioning system.
The sound doesn’t occur if I turn off the air conditioning. If I turn the air on WITHOUT the Recirculation option on (in other words, air is being pulled in from outside), then the clicking doesn’t happen. But as soon as I turn on the inside-air-recirculation button, the clicking starts.
I’m not very familiar with how the S40 handles air circulation, but my thought is that maybe a baffle or vent door isn’t working properly. The clicking/popping sounds a lot like what you would hear when a plastic gear or sprocket is slipping.
Anybody have any clues as to what’s causing this? I’m hoping it’s something I can fix myself, because if Volvo charges $57 for a replacement cup holder and more than $300 for a new headlight assembly, I can’t imagine what an in-dash repair would cost. 
Thanks in advance for any tips!
It sounds like it is the control system of the blend doors. It needs some disassembly to get to it and it may require the replacement of a vacuum controlled blend door actuator.
There are other possibilities but I suspect this is the one. You don’t need to go to Volvo. Other mechanics will work on them, often they are better than the mechanics at the dealer and almost always charge less.
It is not impossible to DIY but this one can be difficult, getting to it. Once there it is usually easy to fix.