Thumping sound/knocking noise when rolling out

Hi All,

I have an issue that's getting worse. Since I purchased my Def 110 97' 300Tdi in 2019 I now and then have a knocking/thumping sound coming from, what I believe, around the transfer box. It doesn't occur regularly and only under the following circumstances:
- Engine hot (driven for at least an hour)
- No gear engaged/clutch pressed
- between 25kmh-0km/h
- thumping gradually gets worse from 25km/h to 0km/h.

Example: have driven for a while, traffic light approach, shift back to 3rd, around 30km/h I clutch and let the car roll till a halt. Thumping starts around 25km/h and gradually gets worse. If I put it back in gear while rolling (2nd for example) thumping goes away (tension on drivetrain back?).

brief background
For the first 3 years this happened occasionally and only lightly. Last 3 weeks it's getting worse, but again not always. Happens both on Low and High, and with Diff engaged as well (did a test). Last week new symptom occurred; after having the car parked for ±5 min after a long drive, when driving off the thumping started vigorously in 1st and 2nd gear (the whole car shook). During normal driving no issues at all. Since March I'm on a world trip and I have been driving more off-road and thus used my transfer box more. Yesterday I have visited a garage but the mechanic could not find the cause (plus reproducing the worst symptoms wasn't possible). We checked:
- U joints
- Wheel bearing and tires
- Propshafts (play etc)
- Drained transfer oil and renewed. Except for 1 tiny 'copper' flake no debris.

There is more play on the front prop shaft than the rear, but both the mechanic and a knowledgeable friend of mine think this should not cause the symptom.

I will check the gearbox oil today for debris, but the Def shifts gears like a charm. The Clutch has been renewed in aug 2021 and clutches smooth.

I attached 2 video's where you can hear the thumping sound. Vid 1 is in High/no lock; thumping slowly kicks in and gets louder. Vid 2 is on High/diff locked and you can hear it while pulling up.

Any feedback - things to look for - comments are more than welcome!

VID 1


VID 2

Might be worth having a look at the handbrake mechanism if it's still fitted. Assuming you haven't changed it for one of those disc brake affairs. I've had noise from the handbrake before, and it was cured by new handbrake shoes. I think it's caused by the friction material slightly separating from the metal part of the shoe and squeaking or flapping about and juddering on the drum. You could try a short drive with the handbrake drum off, to see if you still get the noise. Obviously not anywhere where you need to do a lot of hill starts! Another thing that might be worth checking is the ball bearing races which hold the output shafts of the transfer box. These can become a little slack over time and allow the output shafts to wobble about as they rotate. It's possible to change these without taking the transfer box out of the vehicle if they're loose.

As above hand brake sticking just for a moment then letting go then sticking again and so on. Cause can be the hand brake expander worn/gummed up and not letting go all the way resulting in some drag on the shoes and symptoms you describe. A hot drum after driving is a sign of this.

does it stop when clutch dpressed

Hi Guys,

Thanks for the replies and sorry for the delay, internet was scarce. Removed the handbrake and have been driving around with only the drum installed. Turned out the handbrake caused the problem. One of the previous owners did a ****ty repair job. Found welding work on the top plate that sits between the brake shoes, and the spring retainers on both brake shoes (that slide over the little pins) were missing. Since removed no more issues! Thanks all!

Thanks for the update, not everyone does.