2008 Chevrolet Spark Hi Just Bought Used Car Dealer Singapore That

Hi, I just bought a used Chevrolet Spark from the used car dealer in Singapore. The problem that I hear a rattling sound whenever I engage the reverse gear. I brought this to the mechanic but upon the car engine heated, the problem seem doesn't suffice when I engage the reverse gear. However, we the engine is cold, the problem seem to arise when I engage the reverse gear.