What sound does a leaking air injector make?
 

This is on a bone stock '76 911S 2.7L that was rebuilt around 50K miles ago. Engine runs great and had a recent valve job. Everything good and head studs tight.

At idle all is quiet; however, when driving as rpms increase there's a distinct noise (like a loud valve noise). Internet sleuthing leads me to believe that a leak around the air injector may be the problem..thoughts?