Whine at low speed - transfer case?

tyler711 said:

Hey all, I have a neighbor with a TJ issue I'm trying to help him diagnose. The Jeep is a 2002 4.0 auto with around 95k.

At low speeds (3-5 mph) there is a low but notable whine - similar to the sound that some manual transmissions in reverse make. The sounds fades at higher speeds, but it may just be harder to hear over road noise. It is particularly notable from a stop.

Trying to eliminate various components, we observed the following:

1. The noise does not happen when the jeep is coasting slowly in neutral, or stopped in neutral
2. The noise is speed dependent
3. With the TC in neutral and the transmission in Drive or Reverse (vehicle not moving), the noise is not present
4. The noise generally feels like it is coming from under the tunnel - so trans or TC area; it doesn't seem like it's coming from a diff
5. The transfer case is clearly leaking either from the fill/drain plugs, or the rear output shaft - the forward facing half is clean and dry
6. He said trans fluid looks fine and is up to level

He's going to check the condition/existence of the TC fluid which seems like the obvious first step, and refill. Does test # 3 above (tc in neutral) narrow things down? In that situation, the trans should be driven and so is the input shaft of the TC, though not under any load. Does that point to it possibly being TC output side internals or output shaft bearing? Is there anything else I should recommend he take a look at?

For some reason he has a spare 231 in his lot, so TC would actually be a good prognosis for him.

Click to expand...

• Output shaft bearing: Since the noise is happening at low speeds and is absent when in neutral, inspecting the output shaft bearing is a good next step. If it’s worn out, it could be causing the whine you’re hearing.
• Internal components of the TC: If the fluid checks out but the noise persists, it may be worth inspecting the chain and gears inside the TC for wear. The fact that there’s a leak suggests that it might not be in great condition.
• Alignment & mounts: Ensure that the transfer case is properly aligned and that the mounts are in good shape. Any misalignment could also cause abnormal sounds.
• Test drive with load: If possible, see if the sound changes under load, like climbing a hill or accelerating slowly in a lower gear. This might offer more clues about where the noise is coming from.