They can be adjusted but the first thing you should do is check them. Lots of things can be mistaken for valves tapping.

Using a feeler's guage set you can measure the clearance between the cam the bucket that contains the shim. If the space is too small or too great you can interchange the shims.

Unfortunately exchanging the shim requires a rather specialized tool unless you want to remove the cam entirely (most easily done when changing a timing belt). Also you have to have the correct shim for replacement that gives the correct tappet clearance.

I believe that IPD used to have a deal that allowed you to use their tool and shims and then return the package to them when you are done.

I have seldom found high mileage cars that are out of specs on the clearance. Once you get some real miles on the engines they seem to stablize and require very little adjustment. There is an acceptable range of clearance but off the top of my head I am unable to provide the specs for you.

I have not looked but I bet there is some good reading in the 700/900 FAQ's concerning valve clearance and adjustment (drop down menu upper right corner- "select link")

As an aside you can not change the "hushers" without removing the cam and then the bucket that holds the shim.

Randy