In my experience i've found the thing that makes the car knock (<10) on start up, is the car's recent driving habits. If i've only been puttering around for a couple of days or the car's been idling a fair bit, on a cold start-up it'll have a go at knocking, and also the first time i sink the boot and make some solid boost it'll knock slightly (<20), and sometimes light smoke. But it's right after that without any more knock seen.
The other thing you need to consider is the knock detection method. A knock sensor is really just a flash-as microphone. And sound is only vibration. The classic is the bloke with a screamer pipe off his wastegate that exits near the sensor, and causes enough vibrations that the engine thinks it's pinging off it's tree every time he hits full boost.
I suffer mildly from a similar type of problem due to my "custom"-spec exhaust. I've had the car tuned and it's just an inherent thing on my set-up.
Also, I find I don't so much hear knock as feel it. I find if the car isn't pulling like it should when I give it the giggle berrys, it's cos it was knocking. Still, a decent tune has resolved most of my problems. A turbo with a tiny exhaust housing is still an issue but..... come on pay day(s)!!