Here's my story...
Crickets every morning when cold. Chirpchirpchipchirp.
Took the timing belt cover off and ran the engine. The noise was definitely coming from the viscinity of the idler but I determined it absolutely could not be the idler itself because the frequency of the chirps was way too slow. Here's my logic...
The engine idles at about 800 rpm, or more than 10 revs per second. The idler diameter is about a third of the crank pulley diameter. That means the idler is spinning three times faster than the crank, or about 30 revs per second or 30 Hertz. If the idler squeeked every time it rotatated, it would "chirp" 30 times per second. Think about what that would sound like. It would sound like a buzz or zipper or a machine gun. Much faster than a cricket. No way would a idler or tensioner sound like a cricket.
If not the idler, what? What moves that slow?
The timing belt itself forms a loop that is perhaps five times larger than the crank pulley. That means it rotates five times slower. Therefore, if it made a noise every time it completed a revolution, it would chirp twice per second. That's about the right frequency! ChirpChirpChirp. But why would a belt make a noise once per belt rev??
Recall that the idler runs on the back side of the belt. The side with no teeth. The side with the big, printed manufacturer logo and part number.
It took a little mental concentration, but I was able to watch the belt run across the idler and synchronize my eyes with my ears. Sure enough, whenever that white blur of a label ran across the idler, it made a chirp. To confirm my theory, I sprayed just a little WD-40 on the back side of the belt, and the noise immediately dissappeared, albeit temporarily.
It's the belt itself that makes the chirp. More specifically, it's the ink or paint of the label. Over time the label must get polished or sticky or something that causes it to squeek as it passes over the roller.
That's why VW is reluctant to change it and merely sprays some belt goop on it. There is nothing mechanically wrong with the belt. It will likely last tens of thousands more miles. It's only "fault" is that it makes an annoying noise.
