Fan is causing noise in the circuit
The fan current goes through the same ground trace as the PSU output. This will produce a voltage across the ground trace. The fan draws pulses of current during operation, so the voltage it produces will look like AC noise. 

As you haven't shown us the whole PCB layout I can't tell how long this trace is, but the part shown looks quite long and thin so it probably has significant resistance. 

You should route the fan ground directly to the rectifier ground or close by (not past the regulator ground). You probably should route the relay driver transistor ground to there too, to prevent noise when the fan voltage is changed. Best place to cut the track would be between the output capacitor and the transistor next to it.

Also try to make the PSU output ground trace as wide as possible, using a copper pour if your PCB design software has that feature.