The clicking sound is where the disk platter head has found a damaged sector and automatically pinged back to the 'off' position. As you keep trying to access that area of the disk, it will keep clicking back to a resting position. You can run a chkdsk (in windows, macs...no idea) which will locate the sector and designate it as damaged. As harddrives generally have spare unused sectors it will automatically assign that area as disk space instead, meaning you shouldnt lose harddrive space, just the data that was on that particular area.
Problem is, it depends how long the scratch is. If its just a small jog you may still be able to use the drive to recover the rest of your data and potentially keep using the drive. If the scratch covers the majority of the radius of the disk platter, then it'll likely keep failing.
There are companies that can still recover data from disks with this kind of damage, but generally its cheaper just to buy a new drive if the data isn't important.