The ticking noise is definitely NOT normal and if possible (under warranty) you should have it fixed or replaced.
I have the same problem with my 20" Alum iMac and I narrowed it down to the HD fan being the problem. How? The only time the ticking noise went away was when I put the Mac to sleep (the ticking noise will gradually decrease in frequency until it disappeared). This told it had to be one of the "moving" parts within the computer...HD, optical unit, or one of the three fans. So I installed smcFancontrol and started playing with the minimum fan speeds and narrowed it down to the HD fan. By setting the minimum HD fan speed between 1935 and 1970 rpm the ticking disappeared (normal speed was around 950 rpm).
I know this does not solve the problem (if I lower the fan speed the noise is still there) but the annoying/distracting ticking is gone. I also do not know what is causing this ticking noise...maybe a wire slightly touching the fan blades as they spin or maybe a defective fan.
I must also add, and this is important, that if I lower the rpms on the fan, the ticking seems to be more pronounced than when it first started...or maybe it's just me not be used to the ticking noise anymore. But if the ticking is indeed getting worst I'm just "masking" the problem, the fan is getting worst and imminent disaster is about to happen at any time.
I would like to hear from any one of you guys that got it fix by a technician opening the machine and not by replacing it.