This I will try!
I have a 10 year old Espace (2.0 lit petrol) with an vast serpentine belt - the noise it makes scares crows at 100 paces. Spray-on lube always instantly stops the noise for about a day or two. Originally I used WD40, but this ate the belt in a year or so.
I now use silicone milk (armor-all) or dabs of (electronic) silicone grease as these can't harm the belt and don't affect the function of any of the driven auxiliaries.
andrew