Hi there!
The only two moving parts in a Sky HD box are the fans and the HD itself, all of which are relatively simple to replace, although the process maybe not for the faint hearted.
The fans are on small plugs so they are easy, providing you can source the correct replacements.
The HD is also fairly straightforward but a bit fiddly.
First thing is to get the special screwdriver needed to open the box, which is available at most electronic stores.
Without going into too much detail, remove the power cable and open the Sky box and locate the HDD which is situated in a plastic frame.
CAREFULLY remove the connections, including the ribbon cable (remember to pull the neck of the connector down first to unlock the cable) and lift the plastic frame out.
Remove the old HDD (it's held in by small screws) and replace it with a nice new 750 Gbyte model from Seagate, Western Digital or Samsung, they all work fine.
Just be aware that the Sky HD uses a SATA drive, not the IDE type!
Reconnect the cables and replace the frame in the box, not forgetting to cover the drive with the small accoustic blanket.
Replace the top of the Sky box and plug in.
Total time taken so far, around 15 minutes . . . .
Now the box will see the new drive and automatically format it to be ready.
This can take several minutes depending on the size of the new drive.
Go have a cup of tea and relax.
I usually wait a good 20 mins before powering up the box again, and that's it.
You won't see all of the 750 Gbyte capacity of course, as sky Anytime will reserve it's portion, but you will get tons more space, and it will be practically noiseless.
If it's still not quiet, then replace the fans as they do get dry bearings eventually and will start causing a racket before they fail completely.
There are several sites on the Net which have a step by step guide with pictures detailing how you do this, if you haven't done it before.