In order to change the thermal paste, for reasons unstated, you had to remove the CPU cooler, and put it back on. Assuming there were no other changes, the logical conclusion is that the cooler was not put back on properly.
If you go back, and take it off and back on again, being very diligent in making sure its seated properly, and the noise persists, then report back that you have tried that, as well as a run down of all your relevant PC components, like Brand and model CPU, Brand and model CPU cooler, etc.