Its not un-common for the thermo paste to dry out. I would clean and apply a good quality paste on. Don't forget to make sure the heat transfer fins are clean on the heat sink as well.
As to WD40 be careful here! Spraying it can create a bigger mess. You don't want it on the fan blades as over time it will collect dust.
I would take the fans out and using a non-lube contact cleaner spay (alcohol based) down the fan blades so they are clean of any oil and dirt (make sure they are fully dry before you power them up). Then using a watch makers oiler put a very small drop of sewing machine oil on the bearing. WD-40 is not the best oil for a high speed fan bearing.
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