Manual Transmission Whirs when coasting down, clutch disengaged, shifter in gear

Question

I have a 2004 Nissan Xterra. When I am at speed, usually in 3rd or so, and I am coming to a stop, I disengage the clutch, brake, and coast to a stop, with the shifter still in gear. As I do this, shortly after disengaging the clutch I hear a grinding/whirring sound that is pretty dramatic as the car's speed falls below the gear's range.

If I recreate the sound, it does cease when I pop it into neutral. I can either disengage or engage the clutch at this point; it doesn't seem to make a difference.

Thinking that I was having a cluth-dragging problem, I replaced both cylinders and put in new fluid (the play in the clutch pedal warranted the change anyways). While fixing a soft clutch-pedal issue, it did not fix the problem with coasting.

I was thinking that the synchro gear was going, but wanted to have a second opinion. Any chance that I can simply avoid leaving it in gear while coasting down and avoid having to get this fixed ($$$?)? What did I do to cause this thing to go bad?

The question simply put is: what is the problem, and can I go for long with it the way it is?

Answer 1

It sounds like one of your main bearings is going out in your transmission, either at the front or the back of the transmission (probably the front). When your transmission is under load (clutch engaged, in gear), you won't necessarily hear it. Load can happen either during acceleration or deceleration. If you are really paying attention and it is loud enough, you might hear the noise for the brief moment when the vehicle goes between acceleration and deceleration, as there isn't any load on it at that point. Why does the noise quit when under load? Because the load will take up the slack and won't allow the bearing to vibrate which is the noise you are probably hearing.

You probably didn't do anything to cause this to happen. With all mechanical things, they can just wear out. I don't know if the Nissan's are prone to this kind of problem, so who knows.

One way you might be able to figure out if it's the front bearing is if, while moving where you'd normally hear the noise, put the transmission in neutral and push the clutch peddle in. If the noise disappears, it's probably the front main bearing. If the noise continues, it's probably the back main bearing.

If this is the problem, describing longevity is problematic. It could last for a 100 miles or it could last for the life of the vehicle. It will probably last until you can get it looked at, but you never know. Remember, this diagnosis is just a best guess from the description which you've given.