Citroën C4 1.6 HDi (2008, 66 kW) – gearbox noise, possibly input shaft bearing

[Mykymauz]

Hi everyone,
I have a Citroën C4 1.6 HDi (2008, 66 kW) with a manual gearbox. Recently I noticed a strange humming/whining noise and I’m trying to figure out where it’s coming from.

Here’s what’s happening:

When the engine is running at idle, I can hear a constant humming noise.

As soon as I press the clutch pedal – the noise disappears completely.

If I move the car a short distance forward or backward, the noise stays the same.

But if I drive a bit further (a few hundred meters), the noise goes away entirely until the car sits parked for a while.

Gear changes are smooth, no vibrations at all.
I also noticed a small oil leak from the driveshaft seal, which I’m planning to replace soon.

Based on the symptoms, I suspect it could be the gearbox input shaft bearing, but it doesn’t sound like a dual-mass flywheel or release bearing issue, because the noise disappears after driving and there are no vibrations.

I’m planning to remove the gearbox with a couple of friends soon, and I’m wondering:
Is the input shaft bearing pressed in, or is it held in place by a threaded collar or sleeve?

Any advice or experience with this would be greatly appreciated!

[RichardW]

Most likely this is a derivative if the BE3 box which has been in use for 40+ years! The input shaft bearings do fail, but seems to be more often on the semi auto versions. It's pressed in AFAIK, but you have to dismantle the box to get at it. There's a good chance it's the release bearing, but you won't know till you get it out.

[Mykymauz]

So, what would be the smarter move: should I dismantle the box, or just keep driving until it starts making a really bad buzzing sound? If it’s the sensor, it seems worth replacing, but if the problem is in the box itself, repairing it might be more practical.

Are this Citroën gearboxes generally reliable?

[Mykymauz]

I decided not to fix the slight noise for now, since it’s not really worth it considering the car’s value. However, I’m planning to use a thicker gear oil. What do you think about using 75W-85 or 75W-90 for this kind of gearbox? Also, is it okay to add transmission additives such as Liqui Moly 1040 or Wynn’s 59406 H.P.L.S.?