I try to remember to run all the accessories on a vehicle after I've done any service or install work. That usually means lights, PA, siren, etc. With more and more public safety vehicles using LED lightning, it's becoming a bigger risk. Most higher end lights are designed to not create the noise, or at least not dump it back into the electrical system.
Would be interesting to know if the LED's are powered off the 24v -> 12v converter, or if they wired them to the 24 volt system. Usually LED's will operate over a pretty wide voltage, so it's probably not necessary.