Strange oscillating sound when decelerating

Question

I got a VW New Beetle 2000 with a 1.9 TDI ALH engine. The gearbox is a 02J/EBJ. The car is 19 years old and odo shows 232400km.

There is a strange sound for a very very long time, it was already the case when the car was only a couple of years old.

-> Click here to hear 5 different samples of the strange sound. The strange sound is at the end of each sample. Each sample is separated by a 2 sec silence.

                               

Conditions to trigger the sound

• Decelerating only
• Only within the speed range 20km/h -> 5km/h. The sound appears only at 20km/h and below, and dies when the vehicle speed reaches 5km/h or less. Then the normal idle ALH engine sound comes back. With my wheels/tyres circumference of 1985.17mm, this means within 168RPM -> 42RPM wheel speed range.
• Only when decelerating slowly, in free wheels configuration (neutral speed selected with clutch pressing against the flywheel).

Sound description

• The sound is of low frequency. Approximately the same frequency band as the idle engine sound itself.
• The sound is of oscillating nature. It oscillates as long as we are within the 20km/h -> 5km/h range, and then the oscillation dies.
• The duration is variable. Sometimes the oscillation lasts 2 seconds, sometimes 10 seconds. It actually depends on how long the car will stay in the 20km/h -> 5km/h range. If you succeed to make the car decelerate very very slowly and keep it in this range for a long time (in free wheels), the sound will last a long time.

Possible sound origins

• Silent blocks on the engine or elsewhere
• Imbalance in the drive axles
• Unstable gearbox output shaft or differential
• Wheel hub bearing

For sure, the sound is intimately correlated to the drive wheels speed but I don't know what part causes that sound. Any ideas?

Answer 1

After a half year of investigation, I've finally found the culpit! It was indeed related to the drive wheels speed (the axles speed).

It was the thermal insulating plate between the right axle bellow and the exhaust pipe (1J0 407 721 B):

This plate's role is to prevent the high temperature of the exhaust from reaching and damaging the bellow of the right axle. Look how they are very close to each other (plate has been removed in this picture):

The 2 screws were loose and because they are located at the bottom only, this caused the top of the plate to hang down, touch the right axle head and dance around it. This happened during deceleration, when centrifugal force and "rotational induced vortex" were decreasing.

Here's the picture after replacing the plate with new screws:

After giving the car a 20min road test, I haven't been able to reproduce the sound, which means the problem is resolved!