I have a 99 lightning, Modifications = Diablo switch chip, Single blade throttle body, 4lb pulley
The problem I have is at speeds between 70 and 80 I get a loud screeching noise coming from under the dash. My mechanic and I have checked the air vents and found nothing.
Has anyone had this problem or have any ideas on what to check for?

 

It is possible that your line for your stock boost gauge is leaking.... there should be a line that runs under there.... i'd check that...

 

had this on my 02.could not find the noise.it did not increase with motor speed.but did with speed of truck.it turned out to be windsheld seal,at the lower seal.this made it sound like it was under dash.

 

My 88 Bronco did the same and it turned out to be the speedometer cable.

 

Thanks. I'll give these areas a check.
Not sure if there is a speedometer cable.
Will check the boost line.
It sounds like the same thing with the window seal, This gives me hope, the noise is driving me crazy.