Sorry to hear about your noisey TiVo box. Can I ask where the buzzing is from? I've checked your details and the hard drive status looks fine and temperature is also but want a clearer idea of the noise it's making. Does it stop making the noise when you turn the box off and if not, does it stop eventually? Please also make sure that the box is well ventilated and dust free.
The sound does stops after turning off the box and mostly seems to come on when something is being watched (at least that is the time when it is noticed) and is intermittent. So I can start watching a program, 5 minutes into it the noise might start, then it can go off for about 15 mins, then it might come back 20 mins later, all without turning it off and on. I am certain it is not the hard drive, I have been working with hard drives day-in-day-out for 25 years and know pretty much all the noises that they can make unless its attached to something that has come very loose, plus I also have two tivo boxes and for the sake of comparison can tell when the hard drive is being used or not, for example you can hear the hard drive stop/start even when the this buzzing noise is on.
I can confirm the are is very well ventilated, but I can't say for sure where the noise is coming from, maybe slightly more towards the right hand side as you look directly at the front