and it is doing my head in...its just had a new rear section on the exhaust so sort of sure its not that but it does it when it's in park and neutral,but today i filled the tank to nearly the top (it was just over half full) and my rattle was still there but a lot less pronouched,could it be a fuel pump???

TIA for any suggestions

sorry forgot to say it a 1996 auto v8

Could be your new eggsauce tapping on a bracket somewhere down the system -

your best bet is to roll under and have a look - make sure the exhaust is cold then you can give it a wiggle - tis the silly things

Perhaps them that fitted it used a hammer in the replacement. therefore you now have a ratting cat or cats.
V8s are a very smooth punning engine so the the exhaust system doesn't move at idle, I assume the rubber hangers were also replaced at the same time, which is good practice to do.