Just to clarify - you have hydraulic lash adjusters for the cam followers in the cylinder heads - this is an overhead camshaft 3 valve per cylinder engine.
Your noise - difficult to know where it's coming from unless you start really digging into it. I have see the cam followers pop out due to oil grade and pressure issues, I have seen camshaft journal wear on these engines too. Can't say I've experienced the need for too many lash adjusters. However, anything can happen. The valve covers should come off and the valve train inspected.
If you have to replace lash adjusters, it's not a fun job. The camshafts have to come off, which means you have a ton of work to do to make sure everything stays in place (cam chains, etc), as engine damage will otherwise occur.
Start with the inspection and then proceed from there.