How much noise should the hard drive make on a MBP early 2011?
None.
The machine should only make noise if the Superdrive is being used or the CPU's heavily used, going by my Early 2011 17".
It sounds like a fan but isn't... obviously. Right hand side of track pad, and covering it, in fact just touching it dampens it to barely audible. Is that normal or has something gone a bit weird? A constant fan noise, which isn't the fan is now there permanently.
It's possible it wasn't installed correctly and/or the case is slightly off so it's resonating the hard drive vibrations.
Make sure you have a internet connection (don't move) and reboot the machine holding the d key down like forever and Hardware Test will run.
Backup your data off the machine, if the hard drive dies under 1 year or three years with Applecare, then go get a new one. else you can replace it yourself with a better one and not void your warranty if you don't break anything doing so. Add more RAM while your at it.
http://eshop.macsales.com/installvideos/
As you know Mac's are made by hand by young Chinese women working 12 hour shifts performing the same function every 7 seconds for only $1.20 a hour. They live 7 to a box room and have to pay 70¢ a meal.
All in all I would say they are very tired, and it's possilbe a few defects made it to your machine.