Noises in my head?!?!

I need some help figuring this out. Every so often I hear what sounds like my brakes rubbing. It’s the unmistakable sound of brake pad on the rotor. You know...that, swoosh, swoosh, swoosh noise.....
So that made me believe that it was time for new pads. My rear pads have about 15k on them but my front pads have about 46K on them. But upon inspection there is still plenty of meat left on the pads all around, about 50% on the fronts and 75% on the rear. The rotors also look good. I can’t pin which wheel if any this noise is coming from since it doesn’t happen all the time.
Should I just change my front brake pads or could this be a different issue with the brakes or does something else make this similar noise like a driveshaft or the flux-capacitor???

I'm going to go ahead and replace my front pads but how do I check a U-Joint for this?

Quote from shorebird

There may be a better way, but if you lock down the axle side of the u-joint and rotate the wheel back and forth there shouldn't be any movement in the joint. Mine had rust trails coming from under the cups. While it's jacked up you may want to try to rock the wheel in and out (top to bottom) and check the hub bearing. Does it make the noise under breaking, all the time, when cornering, or whenever it wants to piss you off!

Basically whenever it wants to piss me off..
My speed only determines how fast the noise is but the volume of the sound does not change with the amount of force I apply to the brakes which makes me think its not the brake pads or rotor. I'll need to snoop around and try some of your suggestions tonight.