Strange squeaking sound

I have an '89 SC with a 5sp. I am dealing with a strange squeaking sound when the car is moving. I notice it when my foot is off the throttle and coasting, but when I hit the throttle it goes away. It's the same sound brakes make when stopping and they squeak but this is when I'm driving. At first I was thinking it might be a front wheel bearing, but I think if a bearing is going out it would make noise all the time. Its hard to determine which side it is coming from. I have been driving like this for about a year and it isn't getting any worse or better. Or could it possibly be my clutch. I know that my clutch is getting weak because from time to time moving from a stop either forward or reverse it shudders and third is not really an acceleration gear like it used to be. If any help could be advised it would be appreciated.

I found both my rear wheel bearings were worn out after 77k. Only symptom was a squeaking sound now and then when driving -and slightly warmer wheels compared to the front wheels. The wheel (bearing) will feel ok until you remove the centre nut holding the shaft.
If you drive with a worn bearing the hub will eventually break...as mine did.

Would this noise normally last this long? It almost sounds like a whistle and why would it go away when I press on the throttle?

I had the same whistling sound when coasting and it disappeared when braking. My theory is that when the bearings are worn the wheel (and brake rotor) angle is altered a bit relative to the brake pads. And this is causing the sound and the extra heat. I checked the brake pads with a calliper and found that they were worn unevenly (i.e. different thk. top/bottom). Maybe I have another problem as well, but it could be an idea for you to check the pads.

I'm a bit surprised that the bearing went and I didn't notice it until the hub broke.