And you are sure that it isn't a exhaust/intake gasket leak where the manifolds meet the head?
Mine had a wistle to beign with and by the time I was ready to change it, it had turned into a full fledged exhaust leak with the "put put put" at mid throttle and the old beater truck "boom boom boom" under load...
Also..it whistled something firece as long as it was making boost.
This is a cheap repair so you might check to see if it is the cause of your noise.
Easiest diagnosis, listen for RPM dependent exhaust popping...
If that isn't it...try looking for soot marks on things...between things...etc.

Brandon