ummm, similar story to mine slim - i had my car fully sorted until written off, got what I thought was a very good replacement and have so far had to replace dizzy, rotor arm, leads, lambda and cambelt so hoping to get this noise sorted asap. Mines booked in Monday for the rubber boot to be replaced so lookinf forward to a post when you get yours back 2moro.
I have tried the trick with covering the exhaust and my car runs on still - pointing to a leak or crack in the system but no garage can find this, instead have suggested this rubber boot (gunna b arounf 80 notes to replace).
Good luck for 2moro mate!!
Andy