6000RPM all the time with no activity - restart helped
Sounds like the symptoms of a runaway process using a lot of CPU. Sometimes there are also valid reasons. If it reoccurs your best course of action is to open the Activity Monitor, and look for something using a lot of CPU time and try to determine if there is a valid reason for this (an example might be you have added lots of little files that are being indexed by mds (the indexer daemon for Spotlight)