After further investigation and a fair degree of frustration, I took the dynamo off again, but this time I swopped it for one that I had recently refurbished (I've got 7 sitting here now!) and VOILLA!! The wierd chuffing has stopped doing it's thing!
What is odd is that the dynamo I took off was recently refurbished (not by me) and, when turned by hand it rotates freely on the bearings and makes no unusual noise...
Beats me!! I sometimes get the feeling my bloody bike has a ghost lurking in it somewhere!
