How so?
It’s ok I found it anyway, it was actually easier to find it myself than ask you. Just a hunch which might have been helpful, but led to nothing anyhow.
Why would you spend time trying to find out anything to help me after berating me nonstop, i’m assuming you found it by just typing Zdos123 MX5.
Straight talking Northerners who’ve taken broken engines apart. They can be as rude as they like, they don’t bother me - as long as I can pick up a few nuggets of wisdom.
Knowledge is power.
Any news 
This has been going on since approximately February 2023 Geoff. So it would have definitely gone pop by now if it’s an internal engine problem.
We still don’t know about the auxiliary belt tensioner/pulley fully, even back in April it was mentioned but what happened next?
To the OP I guess it’s worth replacing said auxiliary belt and associated tensioner and pulley etc, you did mention they were ‘rattly’ back in April.
That’s all I can input, if you’ve still not convinced then you really need to find a specialist MX-5 repair shop for more advice.
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It has not done it once since I took the belt off, have 2 new idler pulleys (no a/c so more than usual), 1 tensioner and a new belt on order from bofi, I’ll keep this thread updated when I install.
I should mention that towards the last few starts the engine was struggling to start due to a low battery so I only got 3 good attempts (when it struggles to start for whatever reason (usually when messing around with stuff, has no issues starting day to day) the rattle never happens)
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Below shows the layout for the belt replacement. Idler and tensioner left, replace bottom right pulley on non aircon cars with a fixed pulley. Last time I did this job it cost £100 all in including belt in parts. Using genuine Mazda parts I guess that price will more than double
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Update 
Currently just waiting till I fix my '5, my clutch went out, will update when I get this resolved, I won’t forget trust me.
Just an update to this thread, currently on 24k miles since the rattle started and it is still running faultlessly, does not consume oil really at all and has not progressed seemingly at all despite heavy b-road driving and autocross (get’s redlined a few times a day every day). At my last oil change i got the oil sampled by millers oil anaylsis and they picked up nothing out of the ordinary (very aware this is far from definitive and oil spectography is limited in picking up larger particles indicating wear) nor was there anything in the oil filter which would indicate further rod bearing damage if that is what the issue is (which i’m fairly certain it’s not).
The parts for a new auxiliary belt system will be on their way shortly and should hopefully get the noise resolved, and if not i’ll just keep driving it, it’s not progressing or affecting performance.
I will keep this thread updated
Another update to this thread for all the people here and internet archive diggers:
I have installed the new aux belt tensioner and the 2 new idler pulleys and so far the noise has not made itself apparent and the car is overall a lot quieter and less rattly, finger crossed it seems to have solved the issues, and is not rod bearings as one poster was insistent on. If it reoccurs i will update this thread
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