Could be false knock. That could be caused by a noisey valvetrain, exhaust rattle, etc... 
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1987 GN Astroroof
Precision 6131BB, BGC 23 row stretch IC, 60lb inj., 212/212 Comp roller cam, T&D rollers, TT Akly chip, Razor's Alky kit, Billet main caps, Speed-Pro forged pistons, port & polished heads & intake, 70mm T/B, Edge 10" 3000 stall L/U, trans built by mikestertwo with PTS parts, 3" THDP, Poston headers, PTS aluminum radiator, etc., etc.
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