I've been hearin' this sound for a few weeks now; from the outside, front/back & i can hear it from inside :?

alright...when im travelling about 15-25k/ph...rolling abit slow, when i accelerate quicker KNOCK! then let go of the throttle fast KNOCK!, it makes that sound from the back & sometimes the front..the sound doesn't play up on me when in driving normally, well very rarely.
& often when im sittin' on the traffic then if i press on the accelator alil/quicker(taking off), i hear the sound "knock!" again its annoying :evil: & it worries me sometimes

do u guys encounter this problem or think what the hell it is
pls let me know my ride is an EG H/B

thankz