2012 A6 BITDI Knocking noise when warm in Drive and Reverse

[Leo84]

Hey everyone,

I'm chasing what sounds like a knocking noise when everything is warmed up manoeuvring at slow speeds in Drive and Reverse, the noise dissappears when put in to Neutral. Both Gearbox and Engine have been recently serviced (the noise was there before and after),  the electronic EGR and Engine mounts are also new. The sound is worse the hotter everything is, (ie after a good blast). I've wondered what actually happens when the selector is moved in to D or R if it relates to anything moving around the Turbo area (boost controllers/waste gates etc) as the sound is loudest when I remove the air box. The car is quick and has no other issues when driven. it happens in all drive modes. Its a TipTronic Gearbox. No codes on VCDS.

All help will be greatly appreciatted. 

Thanks in advance.

[GranolaTTS]

It could be a lot of things, have you checked the prop & driveshafts? Could even be engine-related. If it sounds worse when the air box is disconnected this would indicate that it is related to the engine. Have the swirl flaps been deleted? They can malfunction and become loose & even be ingested by the engine causing all kinds of issues.

[Steve Q]

Could it be an engine or gearbox mount? 

[Leo84]

On 2/3/2024 at 8:33 AM, GranolaTTS said:

It could be a lot of things, have you checked the prop & driveshafts? Could even be engine-related. If it sounds worse when the air box is disconnected this would indicate that it is related to the engine. Have the swirl flaps been deleted? They can malfunction and become loose & even be ingested by the engine causing all kinds of issues.

In my mind, when the engine and gearbox is hot, if its oil related, the viscosity of the oil being thinner would let moving parts move/rattle more freely. If its moving parts warming up, then it could be rubbers etc. Audi were useless. Especially when asked " when the transmission is engaged in D or R, what happens to the rest if the car, solenoids, actuators moving etc etc", I got a blank look 😅. What happened to mechanics actually knowing how things works 👀😅. To answer the swirl flaps, those codes always show up as per previous experience in my older A6 I had. They are good. As for the prop shafts and drive shafts, could you please elaborate on your train of thought. I really appreciate your assistance. Thanks. 

[Leo84]

On 2/4/2024 at 7:15 AM, Steve Q said:

Could it be an engine or gearbox mount? 

Engine mounts are new and good. Gearbox mount would be a question as to does its reaction change depending on heat? Most garages would say change it just incase, I've done that already on other parts, costing loads, but have come no closer to fixing it. Audi's are so expensive 🤣. To me it seems like a mechanical issue/noise as it only happens when the gearbox is engaged and everything is hot. Its instant noise off when in neutral, The torque converter is still turning, could it be the turbo waste gate or something relating to something that activates when in D or R? I'd love to read somewhere what actually happens when the gear selector is moved, step by step. I imagine those documents no longer exist 😭🤣

[GranolaTTS]

Prop and Driveshafts have bearings that can wear out and rattle, and universal joints also. You could have a bad bearing in the gearbox which would explain why it goes away in neutral the ZF8 has a pretty big bearing on the rear output.  Does it only rattle when shifting between D & R or anytime the car moves? Or does it make a noise when you lift off the throttle at low speeds? Example ..  reversing into a space and quickly switching to the brake.

 

What I mean does it make the noise between the drive engaging and disengaging? Or would it make the noise if you were driving around slowly locked in 1st gear?

 

Also does the noise change when changing direction or just backward and forwards..