Bob can report the incident to the local Police, and assuming they act on the report there's a couple of different things that can happen (possibly both!):
1. The exhaust/engine has been modified in such as a way that the Police officer believes it exceeds the legal limits.
If the police officer suspects the motorcycle is in violation of the noise limits found in either the The Road Vehicles (Construction and Use) Regulations or Type Approval Regulations (depends on year of manufacturer which applies) but assuming its a more recent machine then it's likely to be homologated to EURO4 levels which are:
|Engine Capacity (cm^3)
||Sound level (dB(A))
|Capacity <= 80
|80 < Capacity <= 175
|175 < Capacity
They can then either issue a Notice of Intended Prosecution under Sec.42 of the Road Traffic Act or if feeling more lenient (or the increase in noise is unintentional) they can issue a 'Vehicle Defect Rectification Scheme' (VDRS) form that requires the owner to get the defect fixed and certified within 14 days.
In the real world unless the exhaust/silencer lacks the required type-approval marks or is obviously defective a prosecution is unlikely as you have to prove the sound exceeds the limits (and humans are crap at doing this accurately by ear)
2. The Police officer has reasonable grounds for believing that a motor vehicle has been used on any occasion in a manner causing, or is likely to cause, alarm, distress or annoyance to members of the public.
This delightfully vague bit of wording is from Sec.59 of the Police Reform Act 2002 which gives constables the following powers:
(a) power, if the motor vehicle is moving, to order the person driving it to stop the vehicle;
(b) power to seize and remove the motor vehicle;
(c) power, for the purposes of exercising a power falling within paragraph (a) or (b), to enter any premises on which he has reasonable grounds for believing the motor vehicle to be;
(d) power to use reasonable force, if necessary, in the exercise of any power conferred by any of paragraphs to (a) to (c).
NB: A PCSO has the same powers as a Constable in this regard except they can only enter premises when in the company, and under the supervision of, a constable
Since even a blind man on a galloping horse can see that such a broad definition of what counts as a contravention coupled with such extreme powers as seizing a vehicle has the potential for huge abuse there's a safety net in the law that unless it's impractical to do so a warning has to be issued first. This warning is attached to both the vehicle and the driver and stays in effect for 12 months, any future contraventions under Sec.59 during this time (it doesn't have the be the same type of contravention) then enables the powers above to be used.
Vehicles seized in this way are then subject to recovery and storage costs which must be paid before the vehicle is released:
- Recovery £150
- Storage £20 per day
Up to a maximum of £430. If not paid & collected in 14 days after seizure the vehicle will be disposed of.