hi
ca's are hydraulic not mechanical
when cams are removed lifters expand (little spring inside)
you bolt the cam back in and the expanded lifter can takes ages to bleed down and let the valve back onto the seat
if the valve is hanging open the piston can hit it on some motors
you can remove the lifter and squeeze it gently in a vice to push the plunger back in .this will bleed off some oil (point oil feed hole down)
do a compression or leakdown test to make sure you have not bent a valve
usually if its rattling it is excessive clearance
get engine warm
raise rpm to ~2000 for 10 seconds
repeat a couple of times
listen for noise
if no better turn off for a few minutes then repeat
lifters can be a bitch to bleed and may take days to shut up
if oil is dirty it can make lifters noisy
precision do aftermarket lifters for ca18
i would try the proceedure above first
if you still have trouble message me
dazza