Completely removing noise from a recording is almost impossible. Mostly this is because there is *always* a noise-floor you will hit against. And removing one sort of very specific audio hum is also going to be difficult unless you happen to also exactly record just the hum in order to take it out later. For someone to do that they're insanely crazy prepared. And I would say if the Police's audio engineers are good enough to find and verify an existing hum, they're going to spot when someone has tried to tamper with it!

(Interesting semi-relevant side-note: the Necomimi from Neurowear comes in 60Hz and 50Hz versions, even though it's entirely battery operated. This is so it can correctly reject the ambient hum from the electricity grid from the brainwave sensor. The 60Hz one usually doesn't work in a 50Hz country!)

Wade.