I'm not normally a paranoid person but EM radiation seems like it's something that it would be easy for someone outside your house in a van to manipulate.
It should be basically impossible for them to manipulate the low bits of the signal. Then you hash it, which makes any bias irrelevant as long as enough bits are random.
If you're really paranoid, just do this:
$ echo "<bash face against keyboard multiple times>" >> /dev/random