[Pretaco]

Check the repair manual on valve adjustment to get an idea of what it all entails. It is possible to do with the cams still installed, but you pretty much have to have the special tools from the dealer to do that. And you have to have whatever size shim you are going to need to replace the one that wrong.
Basically, there is a shim in between the cam lobe and the top of the valve. The shim sits in a 'bucket' that covers the valve spring and valve. To get the shim out, you have to compress the valve down, and pop the shim out of the bucket. Or remove the cam. Then install a thicker shim. How much thicker? Depends on how far the valve is out of adjustment, the repair manual has a chart to tell you what shim to put in, but you have to know what shim is already in it, and how far the valve adjustment is off.
I think there are between 40-60 different shim thicknesses.

So yeah, I have done it and its a pain in the ass, no matter how you do it.

First I would pop the valve cover off, and check the valve clearances, maybe the sound is something else and all of this won't matter.

Ray.