Fans make a noise from boot is the problem (even before the Apple logo appears) - temperature readings are normal, fan speed too (~ 38C/800rpm ODD). When I use SMCFanControl to increase fan speed, the noise increases proportionally - that's how I figured it was the fan (previously, I had no idea what was making the noise). Great idea though.
I only have access to one AASP in my city, and have to travel for 2-3 hours to get to my nearest Apple Store (and I don't drive...). I'm sure if I were there and *made* them hear it (ambient noise tends to eradicate any noise made by my iMac) I'd get my iMac back to its perfect state. Gah.