A New Rattle?

[pylon500]

The saga of the worn SVX (350klms) continues.
I'm still getting that intermitant rough running that we all seem to get from time to time.
It drops the front right plug, or injector, and sometimes the front left as well really rough!, if I leave it idling long enough, while it hunts up and down between cylinders, it will eventually quit, and when I restart it again the problem is usually gone? seems to be ECU induced
Now I've got a new problem;
Starting a couple of days ago, while idling, I could hear this odd rattling, clunking, knocking sound from the front of the engine.
At first I thought it was my newly replaced power steer pump going again, or maybe a pulley about to sieze, but I now notice it comes more from the bottom of the front of the engine like down in the sump
I had a look at some of the older posts dealing with oil pumps, as I seem to be using a little more oil than usual, and I am really asking "how difficult and expensive is it to change the oil pump?
There was a post from 'NOBODY' in Czech republic with a sound .wav attached, but it wouldn't run
Will the oil warning light give any real warning before the engine destroys itself? my oil light has not illuminated as yet
Wish SUBARU had put in an oil pressure guage like they did in the VORTEX!!
Arthur.