Excessive sound (in the opinion of the tester) can be an MOT fail now. I guess it's pretty rare, but something to bear in mind:
Method of Inspection
During all parts of the emissions test relevant for the vehicle, assess subjectively the effectiveness of the silencer in reducing exhaust noise.
Reason for rejection
A silencer in such condition, or of such a type, that the noise emitted from the vehicle is clearly unreasonably above the level expected from a similar vehicle with a standard silencer in average condition.