Engine slighty rattle during accelerating

So lately I've been hearing what sounds like a slight lifter knock that develops at 1500 rpm to 2700 (I have ultragauge) when accelerating or maintain speed, and it only appears when the engine is at operating temp and disappears when rpm is below 1500 rpm and above 2500 rpm my oil is running at 3900 miles using Toyota synt 0W-20 and it looks pretty clean and at high mark of the dip stick, Does anyone else experiencing this sound?

 

I'm thinking that since we use Thinner Oil it doesn't drown out the mechanical tapping of the cam shafts, that's the only possible cause of the problem that i can think off, the fix could be using slight thicker oil during summer time like 0w-30 and the switch back to 0w-20 for winter.