Why has my CPU started emiting scratching/ whining noises?

My laptop has recently started to make a peculiar high pitched whining/ scratching noise, after taking a good long listen, and testing both my fan and my hard disk, I'm 100% sure this is my CPU that is making the noise.

I'm a little concerned as this is the first time I have heard these sorts of noises from this machine.

Possibly related, shortly before the noises started, I had taken my machine apart to clear it of dust, as it had been having some minor heating issues. I had taken the heat sink off to clean underneath it. During this process, I removed some thermal grease on a plastic insert, as it was caked in dust. Any possible coralation? Would acquiring and applying more thermal grease help resolve this? I was careful to make sure the CPU was coated in the same amount of grease as I had found on it, and I'm 95% sure the heatsink was remounted properly.

I get this noise mostly upon cold boot and the first 10-30 minutes of uptime. If I'm gaming on my device I can't hear it as much, (no the fan/ disk don't drown it out), it drops in pitch a little bit and usually becomes inaudible after the CPU heats up past 60 degrees.

This is one of the most irritating noises I have ever heard, and it is slowly driving me nuts. Any help or insight into why this is happening would be greatly appreciated.

Thanks.