Clutch bearing. You could try using left foot to check the same noise on multiple positions of clutch pedal whereas your right foot controls the accelerator. Of course this is assuming the car is manual.
Sidenote on main-bigun:
What the local mechanics call main are the crankshaft main bearings. These are ball bearings.
What the local mechanics call bigun are the connecting rods big end bearings. These are not ball bearings but bushes with an oil path.
Main bearing sound comes when revving the engine.
Big end bearings rattle when you release the accelerator.
The above obviously is true only for those engines in which the problems have started. If the engine is way past gone then the main, big end and all the other XYZ stuff will be rattling like an empty can full of stones.
Generally these bearings are very durable and not a cause of overhaul. Usually they sustain up to 3-4 overhauls in which common replacements are valve seals, valve guides, rings and/or pistons if there is a re-bore. These overhauls are done to reduce excess oil consumption and regain lost power, not to replace worn-out bearings.
Worn out main & con-rod bearings are usually caused by poor quality oil or overfilling of oil. This means the result is seen soon after an oil change and not over tens of thousands of kilometers.