Section 163 Road Traffic Act 1988 gives Police the power to stop all mechanically propelled vehicles on roads. It's an offence not to stop.

Section 67 gives authorised Constables the power to inspect vehicles on roads. It's an offence to obstruct an authorised examiner.

Regulations 54 to 59 of The Road Vehicles (construction and use) Regulations 1986 cover all the offences relating to the use and maintenance exhausts & silencers, including various EEC directives and British Standards which set out noise limits for various types of vehicles (complicated depending on when your vehicle was built etc).

Reg 97 creates the offence of excessive noise ie "No motor vehicle shall be used on a road in such manner as to cause any excessive noise which could have been avoided by the exercise of reasonable care on the part of the driver ."

An authorised Constable can stop you and test your vehicle for a noisy exhaust or any other defects. He can also use measuring equipment if he has it.