Thanks for the tips chaps, I will check all those. the strange thing about the noise is (before I noticed the battery light and had suspicions over the alternator) it seemed to not be present when the bonnet was off, also when I held the springs at the exhaust collector it seemed to dampen it, so it might be unrelated? Fingers crossed anyway. also, one thing I did think, I did let the car idle for about 15-20 mins before I set off out, and had the lights on and the heated seat? Would that be enough to make a overwhelm the alternator and make it do it can't make up the charge when setting off? I would imagine not, but just a thought I had. I will try all the bits suggested, I'm hoping it's a loose connection as I have a Trackday Saturday, and only have Friday off to work on the car! Do doubt I would be able to sort a new alternator before then