David is right. I'll try to find the TSB I had posted before.
Before I begin..."lifter" is really an erroneous name for the parts in question. They are actually Hydraulic Lash Adjusters (HLA)
Most of the time your HLA's make noise for 2 reasons.
1) Inferior oil filter...I've seen HLA's quiet up by simply using a good oil filter.
2) sludge build up in the HLA's itself...this can be cured by simply soaking the lifter overnight in kerosene or diesel.
On that note...I actually took apart a P90A HLA to examine it and see what the potential problems are. It's a fairly simple design. In effect, you have a pressurized oil resevoir that via oil pressure pushes up on the cylinder (much like a brake piston). If the oil ports in the HLA get clogged then what will happen is that you won't get proper oil pressure within the HLA itself...
If I'd have kept my old engine and the HLA were bad, I was going to experiment withing slightly drilling out the oil ports to make the a little larger to help oil fill up the chamber more quickly. This method was actually put in use by Mitsubishi for their newer style HLA's.
Other than that, the only thing I can see that would go bad is if the walls of the HLA itself wore down slightly, not allowing a good seal between the HLA housing the HLA cylinder. Although, I don't know if that would happen...I dunno...the parts are machine with tight tolerances. In either case, I believe that opening up the HLA ports will help...of course, before I did that, I'd soak them in diesel/kerosene over night.
I have a special tool for removing lifters from the cylinder head without having to pull the cam or go through any extra work, but, I belive you can probably use a big screwdriver or small prybar to achieve the same effect.
If anybody is interested I can post pics of the tool.