1. If my assumption might be correct (that the hum isn't the hard drive) what is in that corner of the machine that could hum?
2. Is it something I should be able to confirm without a trip to the shop?
3. It is likely to be something that may need immediate replacement or could it be more like my neighbour (a minor irritant I can learn to live with).
1. The vibration underlying the hum could be caused by a fan or its resonance, possibly via the heat sink hardware
*.
2. If you have a utility that monitors fan speeds, you may be able to correlate hum with certain speed ranges.
3. Answering that is like a crapshoot: youse pays your money, youse takes your chances...
*)
See step 29-30 of this iFixIt guide. The heat sink is the shiny metal frame angling off to the left, the HD is at the top with its controller board in blue, and the HD fan is underneath the motherboard below the HD.
