pingy creaky noise when excellerate

[divito]

iv just put a full stainless exhaust on my 530i and when excellerate quickly i can hear a creaky pingy noise it wasnt there before any ideas iv had it back to were i had exhaust done but they carnt find what it is .its doing my head in fell like selling it coz of this noise ...has any 1 had they same prob any ideas

[divito]

iv just had some mechanic sit in my car to find this pingy noise and he said its called pinkin its some thing to do with timming coz iv had this exhaust put on its got no center or midle box its just straight thru he said i need it remaped is this right pls help mite keep car if i can cure it