I may be wrong, but IMHO, it sounds like valve noise. Like the old cars where the valves would float on the cam at certain RPMS. Since these are probably hydraulically driven, if the oil pressure is low, there is not enough pressure to keep the lifters on the cams.
I found this in my FSM for assistance in diagnosing where and what the noise may be caused by...
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mults
40th Anniversary 'Z', 312.06HP/260.58TQ (Dynosty EcuTek Tune)
Build Date/Number: 1/27/2010 #706
Berk HFC + CBE, R2C CAI, Clear Bra, Cleveland Z Club
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