engine pinging sound or leaky exhaust or ????

my x5 4.6is has been making (for about a year now) a "pinging" type noise when i accelerate. usually, when i'm passing another car and it shifts into its passing gear. its very subtle and really not noticeable but very annoying to me b/c i can hear it clearly. initially, the dealership thought it was loose belts but now they are saying it is the exhaust.

any pointers???

Mine does the same thing, only it's a stretch limo. I've been told it's the U-Joint...that's about it.