Safe and sound by Roxul, or even just regular Roxul insulation are decent choices - not incredibly overpriced, very water resistant and fire resistant to damn near anything short of thermite.
Few things to consider though...
1) I doubt many people hear a fan and assume it's a grow op. You might just be paranoid.
2) A fan controller, or a bigger opening and a different fan would probably work, and be a hell of a lot cheaper than insulating/soundproofing a room. A TV / whitenoise machine could mask the fan, although might be annoying.
3) If your close enough to strangers that your worried about a fan, you probably shouldn't be growing there at all. Even if you don't want to pack up shop now, give it some real thought next harvest - you can expand your operation and get more security at the same time, either by moving or buying a grow house (doesn't have to be expensive, you can get ghetto houses for 5-10 grand, rural trailors for 10-15) Plus, if you don't destroy it or get it seized, you can always resell it, for generally the same or more than what you paid for it.
Few things to consider though...
1) I doubt many people hear a fan and assume it's a grow op. You might just be paranoid.
2) A fan controller, or a bigger opening and a different fan would probably work, and be a hell of a lot cheaper than insulating/soundproofing a room. A TV / whitenoise machine could mask the fan, although might be annoying.
3) If your close enough to strangers that your worried about a fan, you probably shouldn't be growing there at all. Even if you don't want to pack up shop now, give it some real thought next harvest - you can expand your operation and get more security at the same time, either by moving or buying a grow house (doesn't have to be expensive, you can get ghetto houses for 5-10 grand, rural trailors for 10-15) Plus, if you don't destroy it or get it seized, you can always resell it, for generally the same or more than what you paid for it.