Just changed the heads on my car and when I started my car up I started hearing a knocking noise from my exhaust. Never heard a spark knock before so I am not sure what the hell this noise is. It only happens when my car is idling but after I rev the engine to like 1800 the noise stops. I had stock 350 heads with 76cc cc on my car with 8.5 cr now I have heads with 62cc cc and I calculated my compression to be around 9.5. Would this have an effect on my spark timing? I am putting on some 1.6 rr within the next week and I am going to do all the valve adjustments and such, but this noise is worrying me. Thanks for any help