A knock sounds like....a knock
I have run mine at 24° btdc for over 2 yrs now
It used to mis here and there durring one summer when I had POS bosch platnum +4 plugs in (dont know what I was thinking ??) But with the oem nkg's its no problem.
Sometimes Ill drop it down to 22° on really hot days just to keep it a little cooler, but I have both marked on the dist. so its a 10 sec job.
BUT my gf's 92 doesnt even like 22°, but her car was aquired at 145k mi and has only been treated well for 15k mi now. My car was aquired with 28k but now has 104k on it. The difference is that my nearly 80k mi, while hard miles, where well pampered mech wise.