help!! strange noise coming from under hood

[catrachos_b2500]

hello guys well thes is the problem
when am in 3rd gear past 3500 rpm's in my b2500 mazda it starts to make a rattle noise

i remember i had a old mercury marquis and when it ran low on antifreeze
it made a similar sound but i checked it and that's not the problem. some help plzz. (poke)

 

[applerule]

exhaust rattle is what that sounds like. my 95 626 does it bad from 2500-3500rpms