Don't discount the alternator, it could be a good call, it could be the first sign of the alternator brushes going, especially if mileage above 80,000 miles.
The 12v rail gets very noisy and some bits of kit don't like it. Get someone with a meter to checked the voltage with everything switched on (lights, rear-screen heater, fogs, full fan etc) and the revs about 1500, if the voltage is not stable then the bushes are on their way out. This is not the best test, perhaps someone else can suggest better?
Any other unusual symtoms, occasional stalling, radio not as good as usual are also indicators that the alternator is going.
The alternator warning light is useless for this because as long as the brushes are making some contact occasionally, the light goes out. I've had flat batteries due to alternator failure and the light still didn't come on.
A call at the car electrics place at top of green lane station road (actually old-chester road, queen st, chamberlain st junction) may put your mind at rest.
On the other hand it may be nothing to do with alternator - have you taken the relevant fuze in and out in case it has dirty contacts?
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