Andy turbo impreza

hi people!

i have a 1997 classic shape scooby and ive just bought a forge dump valve for it, the valve is positioned on the right hand side of the engine just off the intercooler.
now it fitted to the car etc. and works and sounds great when the car is given some stick, but when,say ur sat in traffic or pulling off slowly in 1st and 2nd and build the revs up to around the 3000rpm mark the valve starts to make a almost like farting sound and the car starts to hesitate, then once the revs are up passed 4000rpm it clears and its fine!

now also when ur sat at 70ish on the motorway it starts to do it to cos the revs of the car at sat at the 3000rpm mark to!!

took it back to the place i got it from and they put a harder spring into the valve but it still does it!!!

does the valve have a bedding in period or is there somthing wrong with the valve???

any help will be much appreciated cos i dont wanna damage the engine!

cheers andy