Strange rattle when accelerating

my 2000 gen 3 jimny makes a strange rattling sound, like if a metal sheet were to vibrate or something, when i accelerate and go through the rev range, specifically from 2k to 3k rpms, also while stationary at those rpms the engine (or something) seems to be tapping.
The car runs very well, the oil only has 2k miles and the engine never gave me mechanical problems, any idea?

As Lambert says, the heatshield on the exhaust, specifically the catalyst under the passengers feet. One or two of the tiny welds detach and it vibrates like the end of the World. Dogs howl, horses bolt, cats run up trees and people point at you if you even tickle the accelerator.

I know this because it happened on mine.

If that is what it is you can get the battery drill to it and use a couple of self-tappers through the flange to the heatshield and sort it without getting the welder out.

The fixings rot out of the front shield on the manifold too, but the noise isn't as loud.

 

i know the problem isn't the rear catalyst, cause i have a new one without heatshield XD. (but the mechanic told me that maybe one of the hangers was hitting on the frame somewhere)
i thought it could be the front one as well, i'll check and maybe take it off and tighten the bolts again.
i don't know about the flywheel or clutch, i've had the clutch replaced 3k miles ago and adjusted the cable last week. (but sometimes i hear some kind of squeaking when i release the clutch, which goes away when the pedal is pushed)
the rattling sound also doesn't always present itself, sometimes it's there and sometimes it vanishes, for example yesterday i used the car twice and the first time there was no rattle, while the second time it appeared only when it was a little warmer.
 

I checked the heat shield and tightened it, the nuts are solid and it doesn't move.
I took some videos of the noise, from inside and under the hood, it maybe comes from the intake side, but I'm not 100% sure. imgur.com/a/HL4ZxEw
I also noticed a yellow substance in my coolant overflow tank, and under the radiator cap, of which I can't take a photo since the engine's warm. The car had been sitting for 2-3 years before I got it from my nan and the coolant was only changed earlier this year, but we couldn't flush the engine.

Unfortunately yellow deposits in the coolant nearly always means that oil has mixed with the coolant and this is often the cylinder head gasket has gone.

Are the yellow deposits oily?

How many miles/km has the engine done and has it ever over-heated?

You could try taking the spark plugs out to inspect (look for one which is washed clean) or get one of these dye leak kits to check.

I listened to the video, I am not sure what the rattle is, it sounds a bit like valve tappets, but it is very difficult to tell.

Robin

The engine was rebuilt once, my nan doesn't remember for what reason though, it runs perfectly, I'll check if the deposits are oily. In the years I've driven the car it has never overheated, even while driven hard and with the old coolant. The first time I changed the spark plugs they were normal.
The engine has 172k Kms/ 110k miles and I don't know how many it's been since the rebuild.
The exhaust smells perfect once at temp, it doesn't smell like gas or steam.

As for the rattle, I think it's that too, it sounds like it's coming from the intake valves or something, I'll check the clearances once I get some tools, I already have the rocker cover gasket.