If your machine is tremendously dusty it's possible that the dust is burning up from being in contact with the hotter components. Another possibility is that a capacitor is 'exploding' very slowly (releasing built up gas from a small leak in the cap as opposed to exploding). If it's not dust then you should power off and unplug your computer for about 30 minutes minimum, and then open the case up and closely inspect all the capacitors you can see on the motherboard and PSU and check to see if any look like a shaken up soda can (bulging outwards from built up pressure). If you find a capacitor like that then you should immediately replace the component it's a part of.