Walk Behind Lawn Mowers

Can someone answer this please, i just purchased Husqvarna HU800AWD Honda Eng. and every time i release the the throttle and then engage the throttle it has a rattle/gear sound once engaged then the sound stops. I returned it back to Lowes and got a new one and the new mower makes the same sound also.

thank you
Jojo

jojo@zoltun.com