How can I track where this 471kHz noise is coming from?
Edit: Upon reflection I don't think this is the culprit in this case, I think Dave Tweed's comment is where the issue is, but I'll leave this in case someone runs into a spikier noise problem. 

I suspect that a switching power supply is causing the problem. Here is the schematic (from Jan Panteltje) of a cheap eBay supply. 



It's not perfect, Jan got the 1M connected to the MOSFET source, it actually goes to +300V. 

Anyway, the point is this thing is kind of a self-oscillating flyback that runs at about 500kHz, with zero EMI considerations. It would certainly fail FCC testing, and hence is actually illegal to sell in the US. No common mode filter on the input, no shielding etc. It drove all his instrumentation up the wall. 

Good supplies can still cause some interference, but if you have something like this it's like setting your 'scope up next to a radio transmitter. 

If you have such a supply, see if the noise is still there with the Arduino turned off but the supply turned on, and if it goes away when the power is cut you've found the source.