One of the things or all:
- you need to clean the PC's case and fans from accumulated dust, ensure proper airflow; dust off the GPU radiator, too.
- make your own fan speed curve in any appropriate software so that you don't hear abrupt spikes in fan speed and constant steady air is supplied to the GPU. But it is not a solution for the big cooling issure you might have.
- you need to apply new thermal paste on the GPU
- your ambient/room temperature has risen so has the max temp - you'll need more/better fans to provide cool air
- you're playing a new demading game and your cooling is inadequate for those loads - you'll need more/better fans to provide cool air
Feel free to ask guidance on any point or google yourself - there are lots of guide online.