My engine makes a horrible sound while above 6K rpms. It is hard to describe the noise, it's a shuttering sound like there are loose marbles within the engine. Maybe this is similar to piston engines when they ping. I suspect it is pre-ignition.

Other factors:
It doesn't always happen.

It seems to mainly happen in 4rth gear. It hasn't happened in 1st or 2nd.

The engine is a streetported s4 na with s5 internals, using a Microtech LT10. (It has been tuned on a dyno and this noise was reported to me by the mechanics who worked on it. I don't believe it's due to bad tuning because I vaguely remember hearing the noise prior to the installation of the microtech. The shop is top notch with lots of experience.)

Please suggest all possible diagnoses and fixes.

 

Classic detonation symtoms.
You have a near fatal engine.
Do not drive in that zone until it's fixed.
If that tuner dosen't recognize rotary detonation, you need a different tuner.

 

I'm using 87 octane, I suppose it would help to switch to a higher octane. This is an NA. I didn't think NA's were prone to detonation.

 

oh yea they sure can, thats how i lost my first engine, yes its rarer in nas but still possible, switch to high octane and i would consult a rotary shop, because the noise you described does not sound good

 

i had a small detonation problem that was only noticable when climbing hills. turned out just to be a timing problem. from what i've read the n/a 13b engine was made to run on 80 octane gas. so if your filling up with 87 your good to go. if you premix then you getting a lower octane rating.