[RichardJ]

Someone mentioned counting the number of threads exposed above the adjuster nut. That is always a good idea.
If you think the noise is coming from any one or more valve train assemblies, Run the engine and grab ahold of each individual rocker arm with your fingers. Oily mess, but it won't bite you. You will feel the noise, in sync with the sound you are hearing.

>> a socket does not fit around the nut when the rocker is in the valve closed position.<<
Are the rocker arms binding? Did you check rocker geometry? Did you check push rod length?