You may have a disk spinning up and down.. put your ear right on the TC.. listen carefully.. if everything is off and the TC is still spinning the disk up and down it really is going to kill the disk.
It could also be spinning up and down the fan.. but the fan will not have the vibration that the hard disk has.
How old is the TC?
There is nothing which can buzz inside a TC.. that is working properly. Feel it for vibrations.. if there are none and the TC feels cool enough.. it could be a faulty power supply.. but I would still think something is causing it to load which is doing it.
Shut down the TC
Disconnect the TC from the network..
Power it up with nothing connected.
At turn on the fan will run for about 10sec. Listen carefully if a piece of something got caught in the fan it will make a buzzing sound. The hard disk spins up after about 20sec. Listen and feel the hard disk spin up.. it will run for about a min and then spin down.. isolated the hard disk should not spin up again.
The TC should make no noise at all in normal standby operation. Neither the fan nor hard disk is spinning.. if it does then take it back to Apple and ask them to check it.