Question about 1990 Nissan Pulsar
I'm afraid you are on the verge of a blown engine, from the description you have given. The loud knocking noise coming from down below is an indication of a spun rod bearing insert, and this is usually caused by either  running the engine with not enough oil in the crankcase to maintain necessary oil pressure,  a bad oil pump or problem in the pressurized oil system causing the same thing, no or low oil pressure, or 
over-revving of the engine, causing too high centrifugal forces on the crankshaft / connecting rod connection.
You need to check the oil level and make sure that is up to full level. Were there any dashboard indications of low oil pressure [oil pressure warning light / low pressure on gauge (if so equipped)]? If there were, you should have paid attention and done the above... if there were none, your oil pressure sensor might be bad.
This is a major repair and requires engine disassembly to the point of crankshaft removal and then have the crankshaft turned and micro-polished by a competent automotive machinist, replacement and clearancing of engine main and rod bearings, and possible replacement of the connecting rod with the spun insert or resizing of the rod and cap, and replacement of the main seals.
If the oil pump is functioning, you can take a temporary measure to keep your car driveable by doing an oil/filter change and refilling with the highest single weight viscosity oil available (at least SAE 50w) and substituting STP Oil Treatment or "Motor Honey" for one of the oil quarts. This should reduce, but not eliminate, the engine knock. Drive very conservatively and keep an eye on the oil pressure, and you might delay the inevitable thrown rod, but try and do the proper repair as soon as you are able. If you do throw a rod, your engine will likely no longer be repairable... You will be looking at a complete engine replacement!!!
Foffy69, there are other ways your problem may have come to be, but for the limited amount of information provided, I have presented the most likely scenarios... Sorry for the bad news, but if you take action now, you can keep it from getting worse.
Best of luck, and PLEASE don't forget to rate my help... this is my only reward for helping you!!! If you have other questions or comments, don't hesitate to post them, or you can reach me directly at firstname.lastname@example.org.
Posted on Jun 23, 2008
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Posted on Jan 02, 2017
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