[help] Gearbox problem (edit: seems to be engine pinging)

[aznalex]

[history]
hey guys, had my car for about a month now..
clutch and gears were perfect when i got it
everything is stock, the only modification is removal of the resonator while keeping the stock airbox in.

[problem]
whenever i shift into low gear and accelerate to medium, theres this loud rattling noise in the engine bay and it sounds like some piece of metal is going to fall/snap off.
as an example, i put the gear in 4th while going at 40kms and just accelerate half way (foot pedal half way) and this rattling noise will start to go off.

[question]
has anyone had a problem with this before or know what could possibly be wrong with my car? i only got it a month ago and this has come up.. a very big disappointment i am facing.

thanks in advance to whoever replies!