Hard Drive "Tocking" noise

Hi all!

Today I attempted to start my computer and was met with a blank grey screen and a "tocking" noise. It would tock about 3-4 times, whir a little bit like it normally does, stop whirring, then tock again. Tocking sound occurs in ~ half second intervals and is easily audible. I cycled the power and it was able to start up fine. This has happened to me once before when awaking the computer from sleep.

System specs are as follows:

Model Name: iMac
Model Identifier: iMac5,1
Processor Name: Intel Core 2 Duo
Processor Speed: 2.16 GHz
Number Of Processors: 1
Total Number Of Cores: 2
L2 Cache: 4 MB
Memory: 3 GB
Bus Speed: 667 MHz
Boot ROM Version: IM51.0090.B09
SMC Version (system): 1.9f4


Hard Drive:
Disk Description : WDC WD2500JS-40TGB0 Media
Total Capacity : 250.06 GB (250,059,350,016 Bytes)
Connection Bus : SATA
Write Status : Read/Write
Connection Type : Internal

I'm running OSX 10.6.8.

I suspect that this indicative of imminent hard drive failure, which considering that it's been chugging along for the last 7 years does not surprise me. I went ahead and backed the whole thing up onto an external just to be safe.

My question is:
Am I right? Is the thing on its last legs? If so, can I replace it easily? I'm comfortable doing simple to intermediate removal/replacement stuff. Pretty much anything short of soldering new components in. Can I put an SSD drive into it?

Thoughts, recommendations, suggestions, and links to relevant information would be most appreciated!

Thanks!
-Will

Yes you are right in your diagnosis. Pop in the SL DVD and run Repair Disk and see what is reported. iMacs are the very devil to work on alas. And yes an SSD will be fine if you get it in.

http://www.ifixit.com/Answers/View/125635/Is+it+possible+to+update+my+iMac+from+HDD+to+SSD