Gents i'm after some advice please

The 3dr has a nasty knocking noise coming from the engine. When cold the car sounds like a diesel! I know that the 2wd engines suffer from piston slap but this seems excessive

The noise does get a lot less as the oil gets hotter but still remains.

If the car is simply idling then i have no noise
If i raise the revs nice and steady even quite high revs i get no noise
If i rev the car with short sharp revs i get the knock!

if i drive the car normal i (by this i mean tootle along like an old lady) i get no noise
If i load the car up (say in to low a gear going up a hill etc), i get the noise! and it increases as the revs do - but only while under load not if i raise the revs steadily.

I'm worried its big ends! But the engine did sit in my garage for 18months while i finished the car. Someone has said that the tappets could have drained of oil in that time and since re-fitting and starting the engine (with new oil and filters of course) the tappets could have gone.

My question is - could the noise i describe be tappets, ie only when under load and not during idle etc
Or is it more likely to be big ends?
Or can piston slap remain even when the engine's hot?
Anyone got any tests i could try to attempt to work out what it is?

I have listened and its hard to say weather its top or bottom, at a guess i'd say its nearer the bulkhead side of the engine, but not 100% sure

it certainly doesnt appear to effect the cars performance

Anyone else had this problem??

Any help is appreciated as i'm ripping my hair out with it at the moment