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I know this is an old thread. But I had same problem. 3 different mechanics telling me different opinions. First mechanic said it's bottom end bearing. Even I knew that a worn bottom end bearing doesn't sound like that. 2nd mechanic said something within the clutch assembly. Didn't make sense as it made rattle noise with clutch in or out. Neutral or in gear. Final mechanic, JDR motorsport from Picton new exactly what it was as it's common within L98, LS3 etc. Sounds like a few 5c coins rattling in a glass jar. It's the timing chain tensioner plastic guides falling apart. And eventually it slaps against the oil sump. And the bare metal of the tensioner. So they pulled off cover and saw that the plastic guides had disintegrated. The chain was slapping against bare metal. And sump. They found the bits of plastic guides in bottom of oil sump. Chain was damaged as a result. While they were doing that, they recommended I go for a racing type of tensioner. No plastic bits. Obviously a new chain. And as the car travelled 270,000km, the harmonic balancer was worn so got replaced with a racing type too. $1850 total later including labour. And the car runs beautiful. Power has returned down in low rev range to the point I'm losing traction. Well done Luke at JDR performance at Picton. Highly recommend for Holden and Ford purists.
Maybe a failing lifter/lifters
Same symptoms that I had. Does it feel sluggish when accelerating hard in 1st gear? Like a hesitation? As I've mentioned in another recent post, my original timing chain tensioner, the plastic guides disintegrated. As a result, the chain was hitting bare metal on chain tensioner as well as oil sump. So Luke at JDR motorsport in Picton told me when they inspected it. The plastic bits were in the sump. I've read somewhere, that if the tensioner begins to lose tension, it will throw out the timing. And the ECU will compensate therefore loss of power. I can honestly say now, since I picked it up last week, the car has come alive! So responsive across the rev range. And no annoying sounds. I had harmonic balancer replaced with a racing type. And I'm not sure on brand of tensioner they put in. But it's a racing type. All metal guides. Reverse concave shape so chain goes around like the cog on a bicycle chain. I reckon either tensioner has broke. Or perhaps oil level is low? Tensioner will lose tension with low oil pressure. Might be best contacting Luke at JDR motorsport by phone. I don't think it's pinging. Obviously you're using 98 RON? Get it fixed asap. Don't want to risk timing chain getting damaged like mine did.