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Sounds slightly asymptomatic to both loose timing gears and rod knocks.
Timing gears are usually quiet(er) on a cold start, and typically fade as the revs rise. Only on a warm idle is there enough slack on the timing gear to let it do a push-pull torque thing as the lifters past TDC. When cold or when revved up, the increased drag from the oil pump puts it into a more constant pull, and it makes less noise without the slack clunking back and forth.
Rod knocks will typically be evident even when warmed up. On a cold start three really isn't more than a couple of seconds of zero oil pressure. And really, a rod that is loose enough to knock doesn't really quiet down too much just because oil is flowing. It wouldn't go away as the motor warms up.
Could be exhaust leaks, those don't typically sound too metallic though. My 1800E had a cracked manifold, and it just very lightly 'chuffed' at idle. A very hard edged chuff sound just might be mistaken for a metallic rap though?
Last thing that comes to my mind (and would match the times yours seems to knock) is piston slap. On a worn engine, the pistons get looser in the bore. They shrink when cold, making the size difference greatest. As they fling up and down in the bore, a loose piston can 'slap' sideways slightly in the bore and make knocking noises. As the piston warms up, it expands, and reduces the slapping noise.
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I'm JohnMc, and I approved this message.
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