I am beginning to hear an irritating vibration noise which I think originates from the exhaust box under the drivers seat
Can you also feel it? I have vibration/resonate point that I suspect is coming from the silencer (Duratec) at about 3k rpm. I can feel it through the whole car. I twigged that it might be the silencer as I have just had a different manifold fitted and that moved the resonate spot by about 500 RPM. Also I had a flexi coupling added at the same time which made it even worse surprising so had that removed. Pondering what to do next but the thought of a little experimental key hole surgery had occurred to me.
