Cyclic noise in drive belt area

Viewing questions concerning different kind of cars I realize that question about this cyclic noise is quit repetitious. It could have any source by any reason. In my 94 Plymouth Voyager this noise is created by evaporate canister purge solenoid, which is normal. At this point I have question to yvesgh: what kind of technique or instrument did you use to determine that the frequency of this noise is approximately 6Hz.

Making a chalk mark on the belt, it was possible to count the number of belt revolutions per second (6Hz is not too high for that) and to check that the cyclic noise is perfectly synchronized with full revolutions of the belt.

I have the same problem with my 2003 outlander i traced it to the little 3x3 metal box in the front left of the car. The box has one line coming in from the bottom of the power steering fluid container and a wire going to the computer. Whatever this box is I am getting it replaced. Before locating the sound I had replaced the belt and the noise continued.