Hi AR,
Obviously you should back up as frequently as you can. However, sometimes hard-drives can go noisy without any apparent compromise in performance or reliablilty.
Up until the mid-'90s I worked as a hardware engineer (not on the PC-platform). The company used to purchase HDs by the hundred from the Far-East, and out of a batch you would always get a few noisy ones that sounded unhealthy. These drives were sold off to employees cheaply, and I still have a SCSI from '93 that makes an awful racket but works perfectly.
As a rule, if a HD changes its "tone" I would recommend replacement pronto, but that doesn't necessarily mean it won't go on for a few months / years yet.