Volvo Repair: 1988 Volvo 240 wagon - strange noise, volvo 240 wagon, wheel bearings

Question
I have a 1988 Volvo 240 wagon (RWD) with 154,000 miles on it.  It seems to be well taken care of, but I've only had it for a year and a half (during which I've driven it 42,000 miles).  
For the past week it has been making a sort of singing or humming noise on the highway, fairly low in tone.  Today after an hour on the highway at around 75mph, the noise became much higher, and I could tell it was coming from the front passenger side.  It sounds sort of like a plane taking off when I speed up and like a plane landing when I slow down - dooOOOO (speeding up) and DOOOooo (slowing down), and like a high pitched singing noise when I'm going at a steady speed (dooo).  Sometimes it's quite loud - especially on the highway - and other times I can barely make it out.  Also, it sounded sort of metallic on the highway.  
I tried shifting into neutral to see if it was the transmission but that didn't change anything.  One person told me it might be brakes (the brake fluid is ok), another said wheel bearings.  It's a cold day and I wonder if that has something to do with it?   (on the other hand, it was warm last week when the problem seems to have started).  
I'm mostly wondering if it's safe to drive on the highway, and how urgent is it that I get it fixed?
Thanks very much!
-Sarah

Answer
Nothing is gonna just break and fall off if that what you're worried about.  I would tend to agree with the wheel bearing diagnosis.  To check, when you turn the vehicle hard and it loads all the weight to one side of the car, if the noise/vibration gets louder then its that side (turn right, load car left; turn left, load car right).  Should only be a few hundred to fix (200-400ish).  Thats the only thing it sounds like.  A mechanic may find something else wrong with it having heard the noise themselves but I'm fairly certain just from your description.  Hope that helps.

James