The 2.7 (3RZ-FE) uses "adjusting shims" between the camshaft and valve lifters to maintain the proper valve clearance. I recommend that you check out the Factory Service Manual (FSM) which was posted by a NORCAL TTORA member to get the proper specs and procedures. The "adjustment" of valve clearance involves measuring the clearance, removing and measuring the thickness of the installed shim with a micrometer, determining what new thickness you need, and installing the new shim. There is a special ervice tool that makes the job easier by pushing the lifer down do you can get the shim out. The dealer is the only place you can get replacement shims if you need them.
Have you checked your oil level recently?
Maybe try switching to a different weight oil too, and get your oil pressure checked. Low oil pressure usually first starts to manifest itself in ticking lifters because the top end of the motor is the part that requires the most oil pressure to get sufficient lubrication.