Okay, problem solved. My keyboard is now consistently quiet in all areas. The problem (if you have it) is that the keyboard is secured in its central section by two screws. You can see where they are by flipping it on the back and noting the two screwholes marked K. This central pressure causes the end sections of the keyboard to lift slightly, so that everytime you press a key down, the weight of your key-stroke bangs the metal on the back of the keyboard against the metal which covers the innards of the laptop.
So I removed the two screws, pushed back the three catches which secure the keyboard and had a look at what could be done. All that was needed was something to be between the keyboard metal and the laptop metal, so I cut up a piece of thin foam of the sort is placed inbetween the screen and keyboard when you by a new notebook. I shaped it to not cover the space where the CPU is, or the memory, and put it in place before re-installing the keyboard. Ah, the bliss of silence.
I was a little bit worried about heat affecting the foam, so I copied a 20GB file to an external hard drive, put iTunes on constant play through the wi-fi card and did a loop of 3DMark01. This got the CPU temperature up to 75, but when I popped the keyboard again the bits where I had the foam were actually still cool to the touch. I may change to some other material, but so far so good.
I don't even know if this would be classed as a mod since the foam is simply held by pressure with no glue (so it is untraceably reversible), but it works great.
Since I write for a living, the keyboard would have started to bug me before long. Now it is just peachy and all is fine with the world again.
Steve