BMW 840Ci, Nreg 1995, 84K miles, 4litre V8.

I have recently developed an alarming noise from under the bonnet (prominent on left side), like a flapping sound. This noise seems to be in time with the revs, i.e. faster and louder as you accelerate and the car shudders at stand still, throttle response seems to be affected as well.

help me out with a little more information
it is difficult to diagnose from what you have posted