Did you happen to install more RAM modules? If you bought 3rd party RAM, it could be that the heat sinks aren't dissipating heat well enough as per Apple's specifications therefore, needing the fans to kick in.
Adding more HDs to the system can also increase the heat and result in the fans revving up.
But as the other poster said, use a can of compressed air and clean the fans and heat sinks in the system.
Monitor the temperature of your system (Temperature Monitor is a good app) and if after dusting out the fans (and making sure that the RAM modules are up to spec.) doesn't help, post back and maybe the other members on the forum who are more knowledgeable can help.