Does this happen in all or some gears? is it worse in some gears than others?
Even though the boost appears to be "steady" the boost control systems (actuator, MAF, VNTmech etc ) goes up and down -
eg - you floor the throttle, more diesel gets sqirted into the engine, the turbine spins up, the compressor spins up, makes boost, higher boost pressure travels along the boost pipes, untill it reaches MAF sensor
Once the desired boost level is acheived (read by the MAF sensor) the ECU instructs the VNT to reduce boost -
This drops the boost pressure level coming out of the compressor, it then travels through the boost pipes/ intercooler untill it reaches the MAF sensor
Now, the MAF reads under the desired boost level, and the ECU instructs VNT mech to increase the boost,
The whole cycle goes round again, normally you are unaware of the (boost control) system doing its job, and it appears smooth
Sometimes, on a heavily modified car thats not set up right, you get this - its called "boost surge" "boost pumping" etc, a lot of it is to do with the delay between the compressor and boost sensor (MAF) the "pumping" can be at a high or low frequency
is it an electronic actuator? is it off the old or new engine?
BOOST PUMPING!! Exacly the term i was looking before i tried to explaing the whole sin-wave thingy :)
The new engine was whitout any accessory, so only block, internals, head and intake. Everything else came from the old engine, and its the 66kw version, that has fixed vane turbo and a wastegate. The actuator for the wastegate is vacuum functioning, but the vacuum comes the vacuum pump via electronic controller. (what is hidden under the HP-pump on the back side of the engine)
I think its most notable when you are drivin whit ~2k rpm on 3rd or 4th, and nearly floor it (~80% of max).
The whoosing sound is similar what you hear, when you try to whistle whit dry lips, and you only hear the air movement :) (that was the closest thing that came to my mind)
I think i need to turn on the data-logging on my OBD adapter to get closer look of the boost preasure. Just incase the gauge it self has some lag and cant keep up whit the boost preasure change.