Engine squealing noises

This noise didn't occur to my attention until today. It happened before, but did not happen in the frequency it did today.

The noise itself is very high pitch and can get louder relative to higher car speeds before breaking 30 mph. It is also consistent as in there are no periods of silence in the noise, just a long stretch of piercing squealing.

The squealing occurs under these conditions:
- After some time of driving
- Happens upon accelerating from being stationary
- Happens if cruising at speeds lower than 30 mph

The squealing does not occur upon starting up the car.

The squealing stops if I step on the brake pedal or if I get to a higher speed.

The odd thing was the squealing occurred only after some time from driving and continued to do so for about ten minutes and then after driving a bit more, the squealing stopped during my drive home. The noise was gone even when I was accelerating from stationary position and cruising at low speeds. Why the sudden squeal and sudden disappearance sort of baffles me.

I haven't checked the car belt, but I did check my brakes last month and they had over 1/3 of it's padding left, so I assumed brakes were not the issue. However, what does concern me is that I can not pinpoint the noise. I had my windows up while driving to see if it was coming from the engine, but the sounds diminished to a whisper. With them down, the squealing noises are piercing.

Right now, judging from my research, there are numerous things that could be associated with this issue. I'm still looking for more perspective on this before I go under the hood and actually inspect this.

Thanks in advance for your time.