What is the mileage of the car and what makes you suspect it is the engine rather than transmission (not saying it isn't just tying to understand more).
Is it a metallic knocking or more a buzzing vibration?
If you can still drive the car get it to the revs/speed where it's making the noise and without moving the throttle change down a gear. Does the noise change? Then accelerate and see if it comes back at the same road speed but with higher revs.
Are you in the UK and, if so, is it with a garage/dealer who understand the S&S?
