So here is the straight answer from another customer with a Rogers PVR.
The hard drive in the PVR will spin down and stop under certain condition which stops the humming that you are complaining about. First both tuners have to be set to a not recordable channel, e.g., 950, so that there is no buffering in progress. Second, and more vague, the PVR has to be in an idle state. This would include not receiving any software updates or interactive program guide (IPG), and not performing a HDD defrag. There may be other obsceure activities.
Very frequently, over the summer, the HDD in my PVR was spun down and therefore quite. But for the last few weeks it has been running constantly. I attribute this to a very poor defrag process that never completes. This seems to happen when my free space goes about 20-40%.
You situation should be different as you do not have any stored programs. Exchanging the unit is unlikely to help.
I would suggest the following. Set the default power on channel to 950. Then perform a hard reboot by unplugging the unit, hold down the power switch and plug it back in. Release the power switch when you see the boot message, and do not use the PVR for a few hours.
Let us know how this works out.