"Police siren" when changing gear from 2 to 3

[smecs]

Hello guys, 

 

I was following and reading this forum for a while when I decided to write because I could not find anything related to my problem. 

To describe the problem: O3 1.6 TDI '13 5 speed manual started to have a noise like a police siren JUST when changing gear from 2 to 3 and only. The rest gear shifts are normal with no strange noise. 

The noise it's the same even if I change gear at 2000 rpm or 2500 or 3000 and so on and appears only if I move the gear stick to change gear. 

In case I'm passing 3rd gear and from 2nd gear I move straight to 4th gear, the sound disappear. 

No sound from 3 to 2 or 4 to 5 or 5 to 4 and so on. 

It doesn't matter if the engine/gearbox are cold or at the working temperature. 

Turbo was replace 100k ago and now the car has 206k.  The car has a stage 1 remap from 30k and this noise started from about 190k and since then it does not increase or decreased.

 

Any ideas to help me find the root cause? 

I used sound dampening materials in doors and floor to reduce the noise and vibrations in cabin and finished with a pretty good level reduction and can't live with this "siren" anymore..

Edited 8 December, 2023 by smecs

[J.R.]

Only during the gearshift or when its under power in 3rd gear?

 

What about downshifting from 4th to 3rd gear?

[smecs]

So the siren appears only if changing from 2 to 3 rd gear and it doesn't matter if I gradually accelerate or accelerate hard and the sound appears ONLY when the gear shift it's about to "enter" 3rd gear. If I'm going with the 2nd gear for 30 km long, no siren.. 

From 4th to 3rd gear no strange noise. Just from 2 to 3. 

[ords]

What happens if you let the revs drop to idling speed?

[smecs]

If I let the rev to drop near the idle rev while I'm with the gear lever in neutral ( for manual gearbox) and then I change the gear, no siren or very little in sound. 

Can't tell why this happens only from 2nd gear to 3rd gear..

Edited 9 December, 2023 by smecs

[Warrior193]

Possibly worn syncro hub in gearbox - do the gears also make a 'clashing' noise when you get this whine? Do you know how to double-declutch when changing up a gear?

[smecs]

Yes, also clashing sound when changing gear. 
Tried the double declutch method and the sound dissapeared when changing the gear from 2 to 3. 
What could be the cause?

[Warrior193]

3 hours ago, smecs said:

Yes, also clashing sound when changing gear. 
Tried the double declutch method and the sound dissapeared when changing the gear from 2 to 3. 
What could be the cause?

Almost certainly worn syncro hubs. These hubs are cone-shaped and act as a clutch to match the speeds of the gear clusters before they mesh together.

By double-declutching, you are matching the cluster speeds without relying on the synchromesh.

I assume you know that the de-clutching technique is different, depending on whether you are changing up or down gears.

[smecs]

Thank you for your answer and time. 

Yes, I ack the technique on YT and saw the diff from upshift and downshift. 

In case of a worn synchro, could it lead to a fail of the whole gearbox if no service ?

[J.R.]

Does the Fabia have the double synchro cones that the gearboxes on the bigger vehicles have?

 

They are pretty much unburstable, in fact its very very rare to hear of worn synchros these days but you never know what abuse a vehicle may have suffered from previous owners.

 

I still cant reconcile a crunch on an upshift sounding like a Police siren, also worn synchros manifest far more on downshifts where the input shaft speed needs to be increased either by the synchro ring or double declutching with a blipped throttle, a slow measured upshift will go smoothly without any synchro as the input shaft speed falls while declutched, it needs precise timing but easily achievable unless you are my niece or my friend in Picardie both of which are very nervous drivers and snatch every gearchange as if the ground would open up and swallow them if they did it it any slower.

Edited 19 December, 2023 by J.R.

[Warrior193]

4 hours ago, smecs said:

Thank you for your answer and time. 

Yes, I ack the technique on YT and saw the diff from upshift and downshift. 

In case of a worn synchro, could it lead to a fail of the whole gearbox if no service ?

No real reason for serious damage provided you were careful with de-clutching - gearboxes were around long before synchromesh became common - but every time the gears 'crunch/clash' there will be a bit of chipping to the gear teeth. Too much damage can start throwing the box out of gear while driving, which is a pain.    

Edited 19 December, 2023 by Warrior193
grammar

[J.R.]

Yes, once a syncro has worn if people dont know how or choose not to double declutch then they overcome the baulk ring function by force and each crunch damages the engagement teeth between the synchro hub and the mainshaft gear resulting in the gear(engagemnt)  eventually jumping out under power or over-run.

 

But that really is a last century problem, perhaps the OP's car has done a strataspheric mileage?

 

I still cannot reconcile the talk of a Police siren noise and wonder if we are way off track from the real problem.

[Warrior193]

To me, the 'siren' noise would suggest a failing bearing - or badly damaged gear teeth - but I would have thought the noise would always be there in that gear - not just during the shift. I was thinking the noise was from the hub itself, as it was trying (and failing) to match gears, but I don't recall ever hearing anything as loud as OP describes. 

Edited 20 December, 2023 by Warrior193
grammar

[ords]

@smecscan you make a recording of the sound?

[smecs]

hello guys and thank you lot for interest in my problem. 

I will make a record tomorrow and post it here. 

The car has a stage 1 from 30k km and now having 207k km