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I've got this strange noise that occurs at low RPM, like below 1000RPM if I'm a little fast off the clutch and the revs drop I get this knocking noise coming from the engine bay. It actually sounds as if the engine is bumping against something. I've looked and I can't see any contact being made, or the engine moving around more than usual on the mounts.

I thought that it might be a bad engine mount, any other thoughts?

Do you know what pinging/detonation sounds like?

Yes it may be getting pinging/detonation due to being under too much load. Try putting the car in 4th gear, drop to 40 km/hr and then try to accelerate, it may do it then also. Like a metallic 'rattling' sound. If the timing has been advanced and there is 91 octane petrol in the tank, it may be pinging/detonating and require 95 or 98 octane petrol. Easy thing to check first.

It could be the Hydraulic lifters.

Have you lifted the bonnet and played with the accelerator and listened for the noise?