Screech at 70mph.

This has been going on for a while now, but I am ready to deal with it now.
When I drive the 71 Duster 318+4 down the highway everything runs just fine.
If i am accelerating, even slightly, at 70 mph I hear a screeching sound. Not super loud but still I hear it.
If I let up on the gas, even the slightest, it goes away. If I add a little gas it comes back. I think once it starts cruising and not under torque the screech stops.

I thought it was the belts, and sounds like it could be, but I have replaced them since this started. I have also used some belt conditioner.
But it makes sense as the torque needs to be involved. But only happens at 70.
and does not happen as the car shifts up to speed and the engine hits the rpm level a couple times already.
I think it may sound like a wheel bearing, but why only at that speed and under torque. A wheel bearing should be sounding at all speeds. also the wheel bearings apparently are only a couple thousand miles old. My mechanic said he replaced them when he did the brakes. I do not always trust what he tells me.

Something else. I wish I could find the video of this. Before I replaced the belts. One of the belts would flip over while the car was running. I would put it in normally and after 30-45 seconds the belt would flip over tooth side up. Replacing the belts remedied that