Engine run in question

[cudamadd]

Hi so we started the 340 to day full rebuild engine run the cam for 25 mins at 2000rpm /2500 rpm not sure exactly. Great temp no leaks all good oil pressure 70 psi 80 and so on . My question is when engine is at 3000 rpm there is a light rattle .we think it sounds like the rocker arms .The break in oil we used was 15/40 Penrith.NO NOISE at idle very smooth and responsive. Basically x heads stock manifolds
After it passes 3000 rpm noise goes away. Its a 69 year 340 . I know on non compression stroke the rockers are a little loose rocker tips are dead centre over the values and so on . Is it normal for a little rattle ? I am thinking that thicker oil will help ,thanks for your time