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RB26 hearing cracking noise/static on knock audio above 6000 rpm

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Have a strange issue while trying to tune a RB26 today while listening for knock on a Plex V2 knock monitor.

Once I reach about 6000 rpm on a dyno run, I could hear some cracking noise in the knock audio which sounds a bit like static. I doubt it is knock though as I can reduce ignition timing by 10 degrees and it is still there. This is not the first time I've heard this. I have worked on another RB26 awhile back (both are GTR R32) and I could hear the same noise. The knock sensor is attached to the cylinder head now while stock location was used on the previous engine.

Does anyone have an idea what the noise could be? I will try and get a recording of the sound tomorrow. It makes me feel very uncomfortable when ever I hear the noise while trying to get the ignition tuning done. Thank you!

I'm betting on ignition noise on your power lines -- how the the Plex V2 powered?

It's currently powered via USB on my notebook. Not sure how I powered it on the previous RB26 I worked on though as it was quite awhile ago. Would the ignition be interfering if it is not powered by the car's electrical system? Or is the notebook itself sensitive to the EMF from the ignition which in turn effects the power that it supplies? I'll try connecting the knock monitor to a separate battery tomorrow.

It the notebook plugged into "mains" power or running off of batteries? Trying an external battery is a good idea.

You might also make sure the routing of the knock sensor wiring is as far away from the coils / HT leads as possible.

Notebook was running of batteries so there shouldn't be interference form the outlet.

A strange thing happened though, I could hear the cracking noise the whole day while tuning under both the 7k band pass and 10k high pass. But when I finally decide to connect every up for recording, the noise disappeared. I did have to turn down volume knob on the unit (the physical dial, not the preamp in the settings menu) or the input on my PC for recording the sound would clip. Maybe I had the volume dialled up too high and the output was also clipping on the knock monitor itself and created the noise? Will try again tomorrow.

I had a problem a very long time ago with an emulator on an old nissan - it would get glitches above certain rpm.

It turned out that the spark plugs were not resistor plugs (eg bcpr7es) and the non resister plugs were causing electrical noise that affected the emulator - but not the factory ecu with an eprom in it - It may not be the case for you but could be worth adding to the list of things to check maybe?

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