I had a similar problem with my HR-20 and it turned out it was the hard drive itself. The temperature sensor is on the hard drive and it (the sensor) malfunctioned. The fan always ran full bore because it thought the temperature was high when, in fact, it wasn't, but shortly thereafter, a DTV update took out the hard drive which may have had issues to begin with (well, it did with the temp sensor), and I decided to replace it (owned receiver). Has worked fine since and I got a big improvement in storage space as well.
Not that this is your case, but it might be.
Not that this is your case, but it might be.