ive got a 53 plate vectra elegance 56000 miles which i bought a month ago. On a long journey to scotland crusing at 80mph i noticed a rattle (like water dripping in a tin can, but metal). it would only happen above 70mph, so i took it back and the garage said it was the heat sheild. They refitted it and the problem seemed to go. the noise dissapeared 4 a while and now again after today getting to 70+ its happening again. i originally thought it was the engine it sounds like its coming from the rear middle of the engine bay and you hear it from behind the centre console area while driving?????

its still under warranty but the dealer didnt inform the repair garage about the noise the first time i had to phone the garage to ensure they knew and they said they didnt look at the engine as they were not told about it (dealer said "noise while driving at motorway speeds") but noticed the heat shield was not fitted correctly and assured me it would of been that making the noise. do you think i should take it back and ask for my money back, this isnt the only fault theres a few but this is the worst. or do i get it looked at again

any guidence or help would be great thanks