Power Loss T25 1.6 Turbo Diesel

[AdyHoggs]

Hi, My van has recently started making a strange intermittent sound almost like when you overwind an old clock, it happens at the end of each gear just before you make maximum Revs, and whilst happening has slight loss of power. It’s strange as after doing this for a few minutes, it stops. But on the next outing it starts again, does anyone have any idea, I have a video with sound, but not sure I can put it up on here, thanks for any help, Ady

[Aidan]

blow off valve on the intake possibly