Could be due to high main bearing clearances. Even if higher than normal clearance is causing the lower oil pressure, hard to say that it would be causing the noise w/o hearing it. Another possibility that I would check on (briefly mentioned in a previous reply) would be valve adjustment. If valves seem to be in adjustment, you could have a "leaky" lifter that is loosing it's prime with the low oil pressure at idle and starts ticking, the pumps up fine when oil pressure increases. Pull the valve covers off (careful, this could get messy) and see if the noise gets louder and/or if you can isolate a particular noisy rocker arm. And, to verify oil pressure, I would pop a live guage on it temporarily and see what it reads. Electrical sending units can be... finicky... at times.