could be the normal HDD (the only moving part in a TiVo) resonating through the shelf it's sitting on, if so try some rubber feet - anything really that would help stop the noise getting to the solid surface, stabilo pencil erasers (nicked from the stationery cupboard at work) will do.
if the noise continues once picked up then it's a louder than number HDD, if you've only had it a short while now's the time to get it swapped - before you start filling it up, new TV season is just around the corner.
Mine rumbles like hell too. Which is ridiculous and unacceptable as I've had dozens of devices containing hard drives sitting in the same unit as my TiVo. Even a HTPC with 4 HDDs wasn't noisy like this. Yes I could hear the hard drives, but not the vibration of the HDDs resonating throughout my entire AV unit.
I'm not saying HDDs shouldn't make noise, they're moving parts after all, but the design of the TiVo is the absolute worst I've experienced in regards to this.
Welcome to the community, I've not been able to locate your account to arrange for action to be taken to take a closer look at this unfortunately, if you've not been able to give the team a call yet are you able to respond to my PM?
You can view the PM by clicking the red envelope on the top left of this page.