weird hiss noise
 

i bought a 98 12v valve 5 speed with 197k on it about 6 months ago and it has a weird hiss/ vibration noise i cant figure out, its not a boost leak, and vacume lines from pump seem fine. it happens ONLY at 1300rpm and ONLY with foot on pedal, weather im free reving it or wide open with a load its only at that same rpm 1300. it doesnt change pitch if im on the throttle more or less. sounds like it toward the back of the motor but i could be wrong, anyone experience this or have an idea, besides boost leak??? thank,