I wouldn't worry about what ran where until you find the leak. You definitely do not want the EGR valve openned when it shouldn't. You are better to have it not work at all. If the hiss you hear is a vacuum leak rather than the normal sound through the idle vale, then this WILL affect your running. We use a number of ways to find vacuum leaks, but in ones that we can hear (sort of) we use a stethescope without the end just an open tube to the earpiece. This allows you to search under and around areas that one's head can't fit.
Bosch Master, ASE Master, L1
33 years MB technician