High pitch whirring sound coming from rear when driving

The other morning when it was colder outside, I noticed squeaking from the rear, when driving. It finally went away. This morning I went for a drive through the neighborhood and heard the squeaking, then I noticed a whirring sound that is normally not there. It was pretty loud.
So my question is, what is back there in the drive train that needs lubrication? I know there is differential oil, but what other moving parts need greasing or oil?

Next time you hear the squeak, press on the brake a bit ans see if it changes or stops. It might be a dragging brake.

The high pitch is usually a motor noise. It probably changes as you go faster or slower. It might be a bearing or maybe one of the brushes,

or even gear whine. When was the last time you checked/changed your gearbox oil?

I’ll check those out.
It never did it before, until the weather changed. I’m not familiar with bearing noise on the Think, but cars make a different noise than this.
I’ve never checked the gearbox fluid. I had it on my list to do, so I’ll get right on that.
Thanks for the input.

Yeah, bearings would make a grumbly noise, but it was turning motor shaft speeds it might be considered high pitch.
If you can run it safely on stands it might help locating the noise.

Gear train would be less, due to it being unloaded(free spinning). Maybe drag brake a bit?

Brush noise would still be there.

Follow Up:
I changed the gearbox oil and man was it black. It looked like it was the original.
This morning I lubricated everything I could, took it for a ride and the noise appears to be gone. I’m thinking the colder dry weather mixed with dry parts, may have caused the noise.
I don’t think it’s the motor, as there is no loss in torque and speed, even though mine may very well be the original 4hp model. I guess time will tell.
Been thinking about replacing it, but keep waiting for it to lose power or fail. It has great pickup and pushes the Think to 28+mph with no issues, and no overheating, even up to 12 mile trips.