Jimny missfire/rattle

Hello There, I got a 2010 Jimny 2 years ago, the car has being doing great and really loving it! some months ago it started doing a strange sound that comes and goes, I dont really know if the correct term would be engine rattling or missfiring, it happens only when the engine is already hot, not when cold like in the morning. happens in 1st, 2nd, 3rd, 4th and 5th, but mostly on 3rd and 4th, but whenever the car has some medium/high load it starts doing it, but again, not when cold. I am using a k&n filter and when I use the regular paper one it suffer a litle more, also, spark plugs and coils are pretty new as I once had a problem with coils. I have been told it could be the timing chain, trasmission was checked about 1 year about as some bearing died and had to replace the broken bits.
Something special is that I have discovered that the rattle goes away if I run the car really hard, going to 6000rpms for some seconds( must be in gear) maybe 15 seconds (rattles hard ) but then it gets almost unnoticable for about 1 week-2 weeks, the only thing that goes trough my head is maybe a valve?
I would really really appreciate some help on this as I don+t think its that common.

Car details would help i.e. engine, transmission fitted. But does sound a bit like a chain. How many miles on it and what is its service history?

M13a engine, stock transmission, got 174k on it. Service has been great, being doing all the oil/filters/ spark plugs, coils have been changed, got a snorkel on right now and k&n filter on, for 5 years was having maintenance at the agency, after that each 5k got maintenance. Diff breathers on to avoid any contamination.

A lot of miles! Try using a stethoscope on the front case to see if you can pinpoint the rattle to a certain area. Obviously be careful of all the whirring bits! Something like a nice long thin piece of metal rod or screwdriver would do. You can use it at different points of the engine too. It would help if you could have a helper to hand to press the pedal to give the rev ranges to try and pin point the noise.

The thing is that unless its in gear it just wont make any strange noise, just rev up like any engine would do but no rattling sound at all, I have tried that but the sound is nowhere to be heard. usually sounds starts around 2500 rpm and up and uphill, almost never downhill, unless I accelerate really hard downhill.