Need some help please

I have a 91 Jetta 1.8 gas. It recently developed a horrendous rattling sound coming from/around the rear passenger seat. The whole pan under the floor rattles and is causing much anguish. A friend of mine is a mechanic and self proclaimed Volkwagen know-it-all. I had him take a look at it and he said it was the fuel pump. $230 + tax and 3 hours of my life lost later, it still makes the rattle. I replaced the original with the same brand and model # part (it's a bosch) and i find it hard to believe that Bosch would send me a defective part. If anyone has any idea what could be causing the rattle, I would appreciate youe help.

Ok, did you check the heat shield above the exhaust pipe? On my 88 Jetta, the metal heat shield is loose and it rattles pretty bad sometimes...

Ok, did you check the heat shield above the exhaust pipe? On my 88 Jetta, the metal heat shield is loose and it rattles pretty bad sometimes...

I fixed the exhaust rattle first thing the whole heat shield is now riddled with self-tapping sheet metal screws. But thanks for replying any suggestions could potentially be helpful