Whistling noise 1985 Dodge B350 while driving 40+ mph

What is your question?

hello, I have a 1985 Dodge B350, with close to (only) 38k miles. Mostly rebuilt (stering, brake system, suspension, as well as new radiator, starter, alternator, ignition timer, etc.), recently I am hearing a whistling noise when on the highway above 40ish mph. Once I get off the accelerator that noise is lower in volume and a bit lower in pitch. I rev'd it up while parkimg and in N shift and barely any or no whistling noise at all (I only hear the normal rev'ing up and like air flow of the fan, etc.). Could it still be a vacuum leak, or more lime weather stripping?

How long has your 1990 Dodge B350 had this problem?

a week