Unless its very noisy no one checks the noise level as is not practical in an operational policing environment to fully comply with all the requirements of the ISO standard 5130:2006 (Acoustics - Measurements of sound pressure level emitted by stationary road vehicles).
Interesting document from Essex police
https://www.essex.police.uk/getmedia/5a ... fences.pdf showing how hard it is to do especially for bikes.
I once asked a Harley owner who didn't have any silencers fitted how he got away with it at the MOT and he said "He just writes slightly noisy on the form! Guess it depends who you take it to