My 98 Nissan Frontier 4WD: When I depress the clutch it makes a humming noise. Does not seem matter whether the truck is moving or not. It started suddenly today after not having driven the truck for 10 days. The somewhat odd thing is that the amount of pressure that I have to apply in order to hear the noise varies. Sometimes as soon as I apply pressure to the clutch I hear the noise and then perhaps as I continue depressing it all the way the noise goes away. Sometimes I start depressing the clutch and there is no noise until the clutch is fully depressed. Other times I can depress all the way without hearing any noise.
Any idea what may be the cause?