cannot hear knock

[yanmar]

Hi, I am unable to hear my engine knocking and don't know what to do anymore. I build up preamplifier for my knock detectors(audi 1.8t AEB) and connected to laptop. The sound is very clear. Later I forced the engine to keep around 1500-1900rpm, started AC and lights  and began to change the advance from 12-40deg. I used bose noise cancelling phones with audacity and filtered the frequenses from 5000-10000Hz. I didn't succee to hear any similar sound to knock box samples from their site. The VEMS manual says to do that exersize at idle, but others say it is very damaging also. I can upload my WAV file if possible, but it is 15Mb.
 

p.s. I bought electronic stetoscope as well, but the sound is not that clear and I don't believe it will show the knock that good.

Here is the file:
 http://www.mediafire.com/?sca76e65lc7nra5

[GintsK]

Forget to get knock with no load! You need boost.  Then add advance in steps of 2 or 3 degrees and hear for knock. It is not damaging if you go from retarded side. Any engine survive few moderate knock events. What is killer - hard knock series with no throttle reliese.

[Ascona 400]

To make it a bit easier, just raise the ignition on one rpm cell. If you have higher ignition on all cells it can still be hard to figure out if its knocking or not. If you know on what rpm you can expect it to knock, it easier to hear it.