Cleaning the idle pulleys.
After a while the idle pulley gets a little bit dirty and don't spin as freely as they could. So cleaning and re-packing them is best way to fix the situation, plus if they're noisey it helps to quite them down a little bit.
Second step is to remove the covers from the bearings. This part is a little tricky. Carefully pry up the plastic ring over the bearing with a small pick or screwdriver. Be very carefull not to damage it. When you done flip the pulley over and do the backside.
Cleaning, the fun part. The first thing I did was remove as much of the grease as possible with a towel. Then I blasted the bearings with WD-40, then towel, then WD-40 ... (repeat until clean)