For some reason, by the time I reach 50 miles/hour (80 km/hour) the noise has already gone down. It could be failing wheel bearings. It could be sawtooth tires.
Can a wheel fly off? I'm avoiding highways (65+ miles/hour - 100+ km/hour) until this is fixed, but are country roads, and country road speeds, also to be avoided?
A noisy wheel bearing isn't generally that dangerous a condition, usually they will just get noisier for awhile. However, if a bearing suddenly fails and takes the rest of the bearings out your wheel could suddenly seize up, or more likely it will get a serious shimmy, which is dangerous no matter what road you are driving on. It would be a pretty extreme case for the wheel to fly off.
You have every reason to act quickly on this. Wheel bearings are pretty cheap to buy and get replaced on most cars. It's a half hour's work for a mechanic with the right tools, and maybe a 1-2 hour job for a home mechanic with basic tools. If you wait until there's a bearing failure and the spindle gets damaged then the costs go up dramatically.
The faster you travel wind noise will often shroud noises that the vehicle is making so don't think that the fault has gone away at that point.
It depends really on how bad the bearing is with regards to what driving it's ok to continue with, really a failed bearing should be replaced ASAP. Some bearings will just hum along for months and months.. But that hum is coming from a damaged/pitted bearing surface, and nobody knows for sure how long they'll really last before the bearings actually starts to break-up/crack etc.
Wheels won't normally come off due to a failed wheel bearing, but they can in some circumstances.. Say an outer bearing race collapses and wears away the stub axle washer & nut, then a wheel may come off.
It doesn't really matter what roads your driving on as a failed bearing, especially a bad or noisy one will start to generate its own heat and self destructive environment, so even driving reasonably slowly won't stop it wearing.
