Hard to describe ,but a sort of twittering noise at 1800 revs when the engine has warmed up .Have just had the car seviced by approved Caterham service place but they had not come across the problem before nor did they have the tools to investigate the complicated bits where the noise may be coming from .No other symptoms and the car runs fine .
I guess I will have to take it to Dartford to have it checked out , but I wondered whether anyone might have any useful comments ?