Maruti Suzuki / Wagon R Duo LXi — GAS -BLASTING SOUND

I Purchased Wagon R Duo (LPG Version) on 15th January 2009. From that day onwards it has some blasting sound while riding in gas especially in lower gears and slowdown . Even thoughI went several times to the service centre, THEY CAN'T rectify the problem, why this happen ? what its remedy any body can pls help me.