1996 k2500 Chevy Suburban 350 small block 5.7 L vacuum leak?

[gages_mommy01]

I hear a whistling sound coming from the front passenger side. I usually hear it when I hit about 35 miles per hour and higher, and it stops when I go below 35. Sometimes I don't hear it at all. What are the possible vacuum leaks? I also have transmission problems. The truck drives around fine, except when I'm going up a steep hill. It struggles to down shift, the engine revs, then it goes into gear. Sometimes after its down shifted, it falls out of gear and does it again. I don't know WTF. Some one PLEASE HELP!!!
A 4L80-E does not have a vacuum modulator it has a PCS, an electronci switch controled by the ECM
yes, no door leaks.