I once experienced the noisy eng start up.. it sounded like the engine was trying to run backwards or severe pre-ignition so here is my theory the engine has 13 to 1 compression based on what ive read. but this is reduced by the valve timing delay. What if the valve doesn't open correctly and the effective compression is 13 to 1 This would be a very good chance for pre-ignition using 87 octane fuel just a theory and im sure there are the engine design folks could verify this.
If you have Scangauge II, check the IGN value when you are getting the knock. It should be a negative number during warm-up. I forget the exact number.
