What could be the source of a high-pitch whistle that comes with long trips? on 2006 Dodge Grand Caravan

The whistle only starts after driving several hundred of miles at a time. Also, in extreme hot temperatures. Other (most) times, there is no whustle at all. This has been going on for two years. It is coming from the back of the vehicle. I assume it involves the fuel system, but I don't think it's the fuel pump.

Asked by for the 2006 Dodge Grand Caravan
very poss the fuel pump itself. have it diag and get est,hard to help without hearing the noise