1HZ knocking noise

Just got back from a trip out through Simpson, Kimberly's, top end of NT and the gulf. The old 80 series rang like clockwork. No problems, not even a puncture.

However the last 2 mornings on the road, it developed a rattle, like a chattering noise, from the engine bay while the motor was cold. Once it warmed up it disappears. Noise only starts about 2200 rpm. Noise is only there under moderate or heavier acceleration. If I accelerate lightly you can go through this rev range without the noise. Happens in any gear. If you load the motor up (or lug it) in a high gear at low revs there is no noise. With the car idling and stationary, you cannot reproduce the noise.

Fuel pump was completely rebuilt only about 30,000km ago. Gearbox and clutch were also overhauled about the same time. Just had new water pump, timing belt and tensioner installed before this trip.

The day before it happened, the fuel filter light and buzzer came on, necessitating the draining of water from the filter. This was done as soon as it came on. Thinking it may be fuel related, both tanks were run to virtually dry, but even with fresh fuel and new fuel filter its still there.

Any ideas on which direction to look.

Thanks

Shane