1979 300d NA 290k

This morning (first frost) I head out and about a mile down the highway
a loud whine starts, after I turn off the radio it almost sound like a bad
fan motor or similar but it is speed dependent.

It’s loud inside the car , the speedo needle is smooth, heater motor is off.

As I slow to the signal light it gets less noisy and it’s gone at stop. As I
accelerate away from the light on the first shift, the noise starts again.

I take it slow and easy as I’m almost at my destination.

As I slow for the turn and accelerate again, no noise. 1/8th of a mile later
I stop.

At first break I’m going to start it up and check the fluid level in the tranny
just in case but any other ideas as to what it could be.

I drove the car last night along the same route and the same speeds
and much further without issue.

To be honest it has sat for about a month since I last drove it.