why do hard drives get louder as they get older?

Yes I am familiar with the phenomenon you describe, but no I don't know why.
The HD in the Imac I am posting from is doing the same thing.
I have a back-up ready and will probably swap it out very soon.
Yes it clatters a bit with normal use and gets louder if it is stressed with several apps at once, or a CPU intensive app. No data corruption but scary.
I don't really trust SMART diagnostics,and drives are cheap these days...
Intersting to hear what others have to say.

Dave

The bearings on the motor the platters are on begin to wear out and cause it to be very whiny. The clicking is due to the heads smacking against a metal stopper wich used to have a peice of rubber on it, but as it gets used the rubber wears out too. Also, the bearings where the heads pivot from wear out.

Exactly right. Next time you have a drive die, take it apart to see what's inside; very educational.

I've found that drive mechanisms from some manufacturers are worse than others in respect to noise. Seagate drives from the last few years seem to be the best at running silently. Maxtor & Western Digital were pretty notorious for noisy drives for a long time, and that's pretty much kept me from using them. Although I did buy a big Maxtor ATA drive a year or so ago, and that runs pretty quietly.

Although Seagate makes a good quiet drive these days, I do have some old Seagate SCSI drives that you can hear on the other end of the house when I spin them up. They came from an old Appleshare server array, and probably have more than their share of miles on them.

Seagate uses and FBD (fluid dynamic bearing) motor, so it doesnt wear out as uick. The old SCSI drive you have probably don't have it. I had a 40GB Maxtor a month ago, bought it brand new, and withing i think it was maybe two weeks the thing squeeled like crazy. Seagate bought out Maxtor, so there shouldn't be any more of that though. I have had mixed results with Western Digitals. I have about ten 500GB Western Digitals at work right now for backup. We'll see how those end up.

I've always liked Western Digital, their drives seem to be quality and seem to last for years. However, every WD drive I have owned starts to get pretty loud eventually.