The WD40 acts as a temporary lubricant, it soon clears completely off the belt, it works just long enough to see if it's the belt or a mechanic issue that's causing the noise.

WD40 is oily - not good for belts! I have an ancient can of Grako Sprazon belt dressing which I use every 10,000m or so at my inspection service. I only ever change belts if something they drive fails, and they have to come off anyway [e.g a Passat GL5 water pump at around 130,000, if I remember correctly]. My record was 242,000m for a GL 2.0 petrol Passat - belt still going strong, engine not wearing as well, but still less than 1l per 1000km oil consumption.

I can't understand why belt inspection and dressing is not recommended - it's far cheaper than replacing a belt which I guess usually looks almost as good as new. Mine do - 118,000 on a 14yr old Audi V6 and 82,000 on a 12 yr old Focus 1.6.

If it works don't mend it! .

I wouldn't have thought putting WD40 on the belt is such a good idea, it's a lubricant, not much of one, but it's a lubricant all the same.

Thats just what is not ,spraying wd40 or variants as described is the correct way to proceed.