System spiking CPU usage (causing 6000+ rpm on fans)
The chart representing the CPU usage in the Activity Monitor can be misleading.

The scale on the Y -axis is not 100% but changes according to the CPU usage.

You could see what it looks like spikes even if they are of extermly low magnitude.

See my example. It looks like spikes while the actual CPU consumption is low. In this case the User- CPU load is 0% to 5.93% and the chart Y-axis is set to 6% so it just looks like huge spikes.