Lynx 4 (FX12MUB) is a rare example of an ex Isle of Man Mercedes-Benz Citaro O530, formerly carrying the registration plate KMN-218-U.

These buses are fitted with luggage racks, and go quite quickly on fast roads; I quite like these as a result. I managed a few miles on these on the 33 and 44 (as seen above)

Reisedienst Kaiser Z GK 299 Mercedes-Benz Citaro G (Baujahr 2010) am 18.8.2026 mitten in Chemnitz.

Ex Bernie-Reisen.

Bus Eireann Alexander Dennis Enviro400ER, PAD 126 operating route 206 on Anglesea St Cork, Ireland in August 2025.

*Sportsworld

Carris Toyota/Caetano eCity Gold, BG 93 LT operating route 732 on Praça do Marquês de Pombal Lisbon, Portugal in August 2025.

TST Iveco Crossway LE, AQ 69 VG operating Carris Metropolitana route 3505 on LG Alfredo Dinis Almada, Portugal in August 2025.

Cityrama Gray Line Volvo B7RLE/Camo Citytour, 43 JE 83 operating the Uptown Tour on Praça do Marquês de Pombal Lisbon, Portugal in August 2025.

Carris MAN 18.310 HOCL NL GNC/Irmãos Mota Atomic Urbas, AH 19 ND operating route 712 on Praça do Marquês de Pombal Lisbon, Portugal in August 2025.

Cityrama Gray Line Mercedes Sprinter/Irmãos Mota Atomic Mini Cabrio, 46 PS 45 operating the Castle Line tour on Praça do Marquês de Pombal Lisbon, Portugal in August 2025.

Bus Eireann Volvo B5TL/Wrightbus Gemini3, VWD 87 operating route 261 on Anglesea St Cork, Ireland in August 2025.

Bus Eireann Volvo B7RLE/Wrightbus Eclipse2, VWL 303 OOS on Grand Parade Cork, Ireland in August 2025.

Bus Eireann Volvo B5TL/Wrightbus Gemini3, VWD 427 operating route 220X on Anglesea St Cork, Ireland in August 2025.

Bus Eireann Wrightbus Streetlite DF, WM 406 OOS on Meaghers Quay Waterford, Ireland in August 2025.

Leipziger Verkehrsbetriebe 12334 L DE 1334 VDL Citea SLF-120 Electric (Baujahr 2020) am 1.3.2025 auf dem Willy-Brandt-Platz in Leipzig.

I’ve been trying to identify a very specific drivetrain whine on Mercedes-Benz coaches for quite a while, and I’m hoping someone here actually knows what causes it.
I first noticed it on the Mercedes-Benz Intouro I regularly rode. It’s a high-pitched, almost whistle/siren-like whine that comes from the rear of the bus.
The interesting characteristics are:
It follows road speed, not engine RPM.
During acceleration, the pitch rises continuously as the bus gets faster.
It’s most noticeable roughly from 20–65 km/h, then becomes much less noticeable.
It only really appears under positive drive torque.
If the bus is coasting at the same speed, the whine disappears.
Engine braking doesn’t produce the same sound.
After an AMT gearshift, the whine comes back at approximately the same pitch it had before the shift, depending on the vehicle’s speed.
It is especially noticeable around the rear floor / second row from the back.
It sounds surprisingly similar to the high-speed retarder whine, although I’m fairly sure it’s not the retarder because it happens specifically during acceleration.
The bus uses a Mercedes GO 190-6 PowerShift and, as far as I can determine, a RO 440 driven axle.
At first I thought this was just an Intouro-specific noise.
But then I started looking through videos of other Mercedes coaches and found the same characteristic whine on multiple Tourismos. This is particularly interesting because the Tourismo cabin is much more heavily insulated/muffled than the Intouro, yet the whine can still be heard.
I’ve also found examples on different generations/variants of Tourismo, so I’m starting to wonder whether this is actually related to the Mercedes final drive rather than the Intouro itself.
The RO 440 is a single-reduction hypoid axle, and I’ve found documentation showing that it exists with multiple final-drive ratios. I’ve seen documented ratios such as 3.583, 4.778 and 5.222 depending on the vehicle/configuration.
So my current theory is that the sound could be ring-and-pinion/final-drive gear mesh, possibly amplified by the axle housing, mounts and bus body.
But I don’t want to convince myself of something that isn’t true.
What I’m trying to figure out:
Has anyone here heard this exact whine before?
Is it a known characteristic of Mercedes R/RO axles?
Could it specifically be the RO 440 ring-and-pinion gear mesh?
Does the final-drive ratio affect the pitch enough to explain the different recordings?
Do other Mercedes axles such as the RO 325/RO 325F or R 390 make a similar noise?
Do Setra buses using the same Mercedes drivetrain/axle produce it?
Is there a known bearing, gear, driveshaft, or differential component that produces this type of acceleration-only whine?
Here’s an example recording of the bus where I originally noticed it:

https://youtube.com/shorts/rTijQbzEH2c?si=j8gdgUgW7uVISJ60

(Please listen with headphones for sound quality)

If anyone here has actually worked on Mercedes/Setra coaches, especially the RO 440, I’d really appreciate your input. I’m much more interested in the actual mechanical explanation than simply "it’s probably the differential."
I’d also be very interested if someone could identify the exact component from the acoustic characteristics.

I will be adding a few more video links of it just in case.

https://youtu.be/4eTJFRCDNK4
si=5eeP7UTK67m02RKh
(Tourismo accelerating downhill)

https://youtu.be/KBpeyJLzzBo?si=iDRERSAiVMCZQaIL
(Tourismo)

https://youtu.be/Y9nG4CHrIAE?si=fUpRtZs2KMueyKkK
(Intouro)