When the airbox wears through the EGR it wears through the plastic cover not the valve it self. From what you are saying the EGR sounds like it is working if the noise goes when the EGR closes. If you remove the air box you should be able to see the pipe from the EGR any leaks should be visible with black soot or oil.
Problem discovered today during an oil change and Air filter swap .
The EGR Valve heat exchanger has broken at the flange which attaches it to the EGR valve. A new heat exchanger and coolant swap will fix that. In the meantime i`ve held in in place with some tie wraps and used some "quick steel" which I`ve liberally coated the joint in. That should last me for a couple of days until the new part can be fitted.