Voltage Spike/Noise due to Switching
If you have a schematic of the circuit and identify where you are measuring, it might be easier to say, but I speculate since you mentioned relays, that this is flyback caused when an inductive load is disconnected.  If you don't already have flyback diodes anti-parallel to the relay coils, try adding that.  

If it is from the relay, you could be seeing the extended and repeated pulses as the contacts separate, essentially bounce.