I've got a wide variety of answers on this ranging from, "It's nothing" to "Fix it now! Don't drive anywhere but to us." Time to finally figure this one out. Okay, here are the details:

The noise has been there since I bought the car over a year and a half or 10,000 miles ago.
The noise seems to come and go. Sometimes it will get somewhat loud, other times it will completely disappear for a little while. Usually it is present.
The noise is louder at higher speeds, regardless of whether or not the car is in gear.
A friend noted that the noise is much, much louder on the right side (where he sat). He moved his head towards me and said it got a lot softer. He also claims it is coming from the right front, but I see no way to verify whether it is in the front or back.
It is usually louder than the engine and road noise when cruising on the freeway. But when accelerating the engine is loud enough to drown it out. It is a humming sound, perhaps something rubbing against something else.
Turning right or braking will make the noise disappear entirely. The noise returns immediately after releasing the steering wheel or brake pedal. Turning left will make it a little louder. Small right turns or light braking will partially diminish the noise.
The car goes straight since I got an alignment a while back.

 

Could it be that your bearings have gone bad?

 

I thinks its bearings also

 

another vote for wheel bearing...

 

my '7 is doing pretty much the same thing, and Syonyk's bet is on wheel bearings for me. hopefully I can at least autocross this weekend without issue...

 

Haha that's what I thought in the 1st place but I heard 4 other opinions so I thought I'd check.

Know any way to tell whether or not it's the front or back bearings?

I already cleaned, examined (all ***** are smooth) and repacked my front right bearings, but only the outer race. I didn't pull the inner race.

SpeedOfLife: well considering how long I've had the noise, I bet you'll be okay. I could be wrong though. I hear the bearings make the noise for a while and then they finally break and do damage to the surrounding parts as well.

 

actually, today we put my car's rear end on stands and drove the wheels and it sounds to both of us like the noise is coming from the differential as opposed to any wheels/bearings. crap. the bearings felt snug, too. it's worth noting that when I changed the diff fluid it was fraught with metal flecks.

so has repacking the bearings helped at all?

 

Wheel bearings are so commonly ignored- until they fail, of course-that it's always a good idea to repack/replace them when trying to chase down a weird, possibly rotationally related problem.
Even if it doesn't solve the OPs issue, it's one more thing to cross off the list and stop worrying about.

 

also make sure all your brake components are lubed up properly. i got a noise like that because i halfassed a rear brake job.

 

Hmm good point. I think I'll replace all 4 (inner/outer) front wheel bearings with OEM units. That way they should be good for another, what, 100k miles or so? I also need to pick up a rear brake hardware kit to replace a missing part or two. Though I think I lost that part, not the P.O., and it's been making that noise since I bought it. And I may look at the front brakes, since I don't think I've ever had to change those yet. After all that I think I'll just take it to a mechanic if the noise persists. But yeah, good point, I should do those things anyway. Or at least sift through the P.O.'s documentation to see if they've ever been replaced. Yeah, he gave me full documentation; a folder full of 50+ mechanic's receipts .