I have a 2007 W24 chassis with the 8.1L engine and had this problem. I checked idler and tensioner pulleys and they were OK. Tried several belts and noise returned in a short time. I tried NAPA, Dayco, Gates and they didn't last very long until noise returned. 5 years ago I installed a Goodyear Gatorback quiet belt 4061080 and it has been quiet. The belt is a cog-belt design with slots on the groove side that are perpendicular to the grooves. This belt seems stronger than others I have tried and streches less. It is just now making a light chirp at idle speed and usually in the morning or when damp. I will have to now check further now to see if I can determine cause. It does not sound like anything is about to self destruct and the noise goes away above idle. This belt is under quite a bit of stress since it runs water pump, alternator, AC compressor, and Hydraulic pump for steering and brakes.
The easiest way I have found to handle belt removal and installation is to attach a ratchet strap to the tensioner arm and the other end of strap to suspension spring on passenger side. This way you can remove tension by yourself and easily work with the belt. Just make sure you follow belt routing. I can remove the belt completely from below with this method.
Looks like I should also take a good look at the crankshaft pulley. Good luck with yours-- Mick