Here's how you can check your heatsink:
Make sure you turn off the machine. flip the switch on the power supply (from 1 to 0) and pull out the plug. Then touch a metal part of the case (discharges static electricity on your body). Then you can start testing if your heatsink is loose.
There are four push pins on the stock cooler. If any of the pins are loose, you should be able to wobble the cooler by grabbing on the sides and trying to tilt it (just slight force). If it wobbles, then one or more pins are not secure. Push down on the loose ones until you hear a click. That means they are in place.
Once it's been reseated, you can put the lid/side panel back on, plug the cord back in, flip the switch to 1, and power up your machine.