Do you have anything running that would try to read all the files and build a search index - like beagle? There's also the nightly run of updatedb but that just reads the filenames and normally nfs mounts are excluded.
There is no package beagle installed, I don't know if any other software doing this is part of a standard CentOS install. Definitely not updatedb, mlocate.cron runs once a day in the early morning, but the load pattern we see is a continuous increase.