I would bet its the hard drive in it, a lot of them come with whatever was cheapest. A lot of Seagate drives (especially those from 3+ years ago) were known to be pretty noisy and have quite high level of vibration.
If you can, you might try and get the DCX3501/M (aka the RNG200N) since they should be only a year or two old at most.
Just a word of warning I've now seen Comcast also call the DCX3400/M an RNG200N since they both fit the requirements
to be called an RNG200N.