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This sounds like pre-ignition. VERY bad if so - you can destroy your top end, or burn through your pistons very quickly if you're experiencing this problem. Bruce Anderson's book describes the problem. The cure is to dial down the boost, reduce the charge air temperature (bigger intercooler), increase the fuel delivered (lower air/fuel ration, or run it richer), and / or run higher octane fuel.
Is there any way you can pump a gallon or two of race fuel per tankful? Can you at least find 92 octane fuel from one of your local gas stations?
Don't ignore it. Find someone who really knows what they're listening to, or at least someone who has an air/fuel meter that you can check out your mixture at high boost with. This usually means a dynamometer setup. It's not cheap, but you won't like paying for a new engine.
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