I think it may be the high pressure pump squealing. Check the cooling water flow through the fuel cell, if it is restricted the fuel in the cell heats up and the high pressure pump vapor locks and it will squeal. At idle the engine uses very little fuel but the pumps put out enough for full throttle, the excess is recirculated back to the fuel cell. At idle the raw water pump puts out low volume, any restriction in the fuel cell cooling jacket will result in insufficient cooling and gradual heating of the continually recirculating fuel. The situation is resolved when you throttle up by drawing cool fuel into the cell. Note: fuel cell cooling requirement is highest at idle and lowest at WOT.
