Possible turbo problem!?!?!

[97f450]

i have a 1997 fsuperduty/f450 it has 141k runs amazing. well maintained. I just recently straight piped it and its ggot a nice deep growl with turbo whine. Well here is the thing im driving my truck and if my foots not into the pedal or i let off. the exaust sounds like a huge air valve releasing pressure then like it automatically closes. when it dose this it drops rpms and reduces speed drastically. i have no clue what it is and im about 100% positive i have no exaust brake

any ideas i need help

[Nitro41]

theres a 2 wire plug under your compressor housing of you turbo. unplug that and see if it goes away. it sounds like you Exhaust Backpressure Valve (EBV or EBPV) is switching on.

is it really cold outside when this happens?