Here's "my guess": When doubleplay was added, there was also a change to where the guide data was stored on the drive. It would seem this could be in the center of the drive and the buffering/recordings are on the outside. Since the DVR is always buffering, it's writing to the outside and then the guide data updates come and this needs to be written to the inside. This causes the head to "bang back and forth" as it is writing to both the inside & outside of the platter(s). Once the guide update is finished, it quiets back down and goes back to writing on the outer part of the platter(s).