In order to save a little bit of money, maximizing profits, it’s common for companies to use cheap thermal interface material. In the case of the PS3, the thermal paste that Sony used for the Cell processor and the RSX graphics processor doesn’t age very well.
I recently bought an original 60GB PlayStation 3, with the CECHA01 model number. In the past I have used newer PS3s, and they were all reasonably quiet, so I was surprised at what I heard not long after turning on my console. The fan went to full speed, and never went back down to a reasonable level.