From:
http://www.direct.gov.uk/en/Motoring/OwningAVehicle/Mot/DG_10016070"Components tested during your MOT test:
Exhaust system - complete, secure, without serious leaks and
silences effectively"
From:
http://www.vosa.gov.uk/vosacorp/repository/08%201246.pdf"With the engine running assess subjectively the effectiveness of the silencer
in reducing exhaust noise to a level considered to be average for the vehicle,
and the Reason for Rejection (RfR) is; 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 silencer in average condition."
"As stated in the Method of Inspection wording, the test is subjective, and therefore relies
on the tester s judgement. MOT testing stations are not required (or permitted) to
use noise meters assess the levels of exhaust noise."
hth