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My believe is that the cause of the noise is that the lifters do not get enough oil to pump up. It is particular so at idle. As you have changed the lifters probably the lifters are not the cause. The likely cause is that not enough oil is getting to the lifters which makes them noisy. Also the lifters gets oil in when the cam lob is on its way up and looses oil when the cam lop is pressing on the lifter. So a possible scenario is that ware in the head will cause the lifter boar to increase thus upsetting the balance of the oil circulation though the lifter and causing the noise. The balance is particular upset at low rpm which is possible.
The solution will be to go for oversize lifters that reduce the clearance between the bore and the lifter. However, I am not aware of such lifters. Another alternative will be to go for solid lifters. This will be fine I believe but very expensive.
Also is possible this type of noise is not actually lifter noise at all. It may be valve guide ware which can be rectified by an head overhaul.
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