Here's the rule from the 2014 CAMS Manual

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NOISE: The Organiser shall have the necessary noise measuring devices in place, and they shall be in operation throughout the entirety of a race meeting.
In order to assist Competitors, after each practice and qualifying session, each Driver of an automobile which records sound levels of between 93 and 95dB(A) must be so advised by the Organiser.
Each automobile which records in excess of 95 dB(A) at any time may be removed from the circuit and not permitted to resume practice or racing until the Organiser is satisfied that the problem has been rectified.
At each race meeting, at least one official must be nominated Judge of Fact (noise level) and his name
included in the regulations for the meeting.

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Peter