What causes a screaming noise from under the base of the inlet manifold at 1500 to 2000rpm on 3.2 litre?

There are no air leaks. The noise can be clearly heard in the cabin at light or zero load. Under firm acceleration, the noise cannot be heard.

The base flange of the inlet manifold contains small air passages that provide air assistance to the fuel injectors. The air is supplied from the upper throttle body through a large vacuum hose that splits into two and connects to each bank at the base flange on the inlet manifold. A small valve on the throttle body controls the flow into the hose. Removal of the electrical plug from the valve makes no difference.
I understand that a small number of other Jag owners have tried to overcome this problem with no success.