Pc shutsdown mid game?

fan makes loud rattling noise for a few minutes before the whole computer shuts down. a program i have says the temperature goes up to 40 degrees celcius at one point. Pressing my foot against the case stops the sound and makes it much less likely that it will shut down. My computer isn't very powerful but it should be able to to play things like minecraft. What would the best thing to do? My computer repairman says nothing is wrong but the sound is deafening.