The motor bogie should simply be replaced and the original faulty unit send back to Fleischmann Germany for examination.
This would be the right thing to do.
This is customer service. Bachmann did it with the Class A1 motor and I cannot think of a Hornby example off the top of my head right now (Scalextric Digital Slot Car Racing Control Units?) but no doubt they would do the right thing also.
Your dealer who you bought this from should organise this for you or give you your money back. Have they said why they cannot do this?
No point in messing about. Ask the question!
I bet if you were next door and not 500 miles away in Scotland you would have had a new motor by now.
Do these folk want your continued business or not? Sorry to be brutally honest buts thats how I am when it comes to customer service. Messing around with issues like this does not do the hobby or the industry any favours whatsoever.
PS I have noted the other thread where you have been given instructions on removing the body. In it there is a claim that these motors are inherently noisy anyway. What I will do is have a listen at the Fleischmann display layout at the Warley show and see exactly how noisy this model is. The DMU that John Hill's had running at the Ontracks Continental Open Day was super silent. It had to be as John was demonstrating sound!