The old high floor buses had modified axles to ensure a smooth ride along the track. The fleet that operates along the O'bahn today is just rubbish purchased at the cheapest price that was available as a result of the tender, and they have had no such modifications. Quite a few accessible bus chassis types have been trialed on the O'bahn, and the government for some reason seemed to just basically give up and order off the shelf vehicles.
In the last 8-9 years, they have been subjected to many engine/diff/gearbox issues and there have been at least 2 in the last 3 years that I have personally observed as being unsuitable for use due to issues with the rear axles, and vibrations from the gearbox. Also, quite a few O'bahn buses have developed severe axle wear.
A rumour I heard indicated that it could be one reason why Broadspectrum left the scene very recently, as they were losing a lot of money due to the cost of repairs to the existing O'bahn fleet.
One common cause of excessive noise from inside the vehicle are the differentials. At least one bus has even had a computer failure or something wrong with the electronics (recently), leaving the bus to remain stranded with no way out for the innocent paying passengers. Several have caught fire with passengers aboard, which has on at least two occasions sent the respective vehicles into very early retirement.
All of this will inevitably reoccur over the years to come if the current fleet remains operational without any modifications.
I know this is going off topic, but it should answer your question.