Grinding Noise when accelerate / increase revs!

[mcyates]

Noticed it last Friday when driving in heavy traffic on the M1. I accelerated and there was a kind of grinding noise coming from the engine bay, got louder and more high pitched the higher the revs, sometimes if I revved high to accelerated it would stop, then go a way for a bit, then it would do it sometimes and then not other times. When It did do it, as soon as I turned the air con off, it stopped the grinding, I turned the air con back on and it started again.

Could it be a problem with the Air con clutch or bearings?

Thanks in advance!