Bent valve doesn't explain intermittance described by the OP.
Bent valves also make noise through the entire powerband. I agree that it's a distinct possibility, but I'd look for heat related ignition and fuel system issues first due to the intermittance. If you want to try and rule out detonation/pinging, then you can always try and fill up with premium and add octane booster. The added octane would, in most cases, cause the nature of the sound to change if it's at all related to detonation.it only does it when it been sitting for a while and i start it after its cooled off
Actually you know what? I think I just ruled my own suggestion out. Detonation would occur more commonly when the engine has been warmed up for an extended period of time and not allowed to cool. I would start looking down the route of oil-starved moving components. Does the sound go away after the car has been running for a while? Oil-starvation can cause a "knocking" or "tapping" type of sound. Not sure why it wouldn't do it on cold startup as well though.
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