Thanks for this useful info. I will check again. Significantly I had some earlier trouble with the rear section which forms the exhaust pipe. This has a liner of pierced metal plate. This was vibrating at certain revs and I managed to cure it with an injection of a small amount of Araldite.
The 'new' sound is different but it may well be from the same source but further up the pipe. I will have a closer look and maybe take off the units to give them a shake as Dave suggests.
The idea of securing the liner with bolts sounds good if the problem is there.
Much appreciate your response. Best regards.
