are you sure is a valve noise?? or is a piston slap?
Slap it around
With a new valve gasket
Remove the head and have the valve reseated or replaced.
It all depends how the tuba valve is broken.
The car code p0446 is a faulty vent valve. To fix this problem, the vent valve will need to be replaced.
Replace the valve seal and check the valve guide for wear.
You don't fix it, you replace it.
The only way to fix piston slap is to replace them. Some oil additives will quiet them down some, there are several brands available.
Shut the valve below it
replace the egr valve
Go to youtube, search for Davidsfarm, and he shows how to fix a valve.