Sound seems to be coming from behind dashboard

[Victor]

Just yesterday I started hearing a sound that sounds something like a screaming electric motor bearing or whistling from an air or vacuum leak, hard to describe. It starts at about 35 MPH while I am accelerating, and goes away after a minute or two. If I decelerate or drop below 35 it also goes away. I can't hear it if I open the hood up and step on the accelerator while standing still. It seems to be coming from somewhere behind the center of the dashboard. Any ides what it could be. Are there any vacuum lines behind the dash, like to control the heater or vents? It doesn't sound like the squeal from a loose drive belt.

[GerryO]

EGR valve/connections?