This same problem just surfaced in the '86 Graduate I had loaned to a co-worker. Squeal to beat the band at idle, then much less when the manifold pressure rose as the throttle opened. Your problem noise might also be related to manifold pressure instead of anything else. There is a small tube connection at the lower rear corner of the intake plenum. On our car, it is a fabric reinforced rubber tube which mates up to a yellowed vinyl or polyethylene tube routed through the firewall and apparently on to the back end of the car. When I pull this connection from the plenum, the sound ceases.
I could identify none of the three small boxes adjacent to the ECU as being the culprit -- certainly not the Bosch connectored black and round-shouldered box nor the multiple pinned relay(?) box with square cross-section and about 3-4 times that long at the base of the ECU, and it didn't even sound like the problem came from the third and small shiny metal relay(?) box of roughly cubic shape at the upper front of the passenger side of the ECU. I used a piece of rubber hose as a stethoscope and couldn't find the source of the sound, even after pulling up the package shelf.
There is an apparent extension of the yellowed but still mostly transparent tubing that emerges from the structural channel on the passenger side of the car, along with the ECU wiring. It appears to snake across the back of the car and enter into the trunk to join up with the vapor cannister at the front of the trunk. The tube joins with a possible one-way valve before going to other things. I didn't disconnect it in front of the valve, but did disconnect it on the cannister side without any effect on the noise. I could see no connectio of this tube to the ECU or its attendant small boxes. They seem only to have electrical connections.
I am now at work and without my manuals, so I can't look up where the various items are. Would anyone have some pointers and suggestions? I am considering the possibility that the vapor return line has started whistling from a leak somewhere, possibly from the one-way valve body itself. But I am taxed for time, tools, and text (okay, poetic license, but the meaning is there and "text" _does_ alliterate better).