Engine/Turbo noise- can anyone help?

Hi, I have noticed with my Bora (its a 115 TDi PD) that when the revs rise I get a very noticeable harshness/vibration at approx 1800 rpm. If I sit with the car idling in neutral (foot on or off the clutch) and slowly rise the revs I can hear the enoigne get louder and vibrate/boom the car at this rpm, and then continue on up the rev range. It only stops if you let the revs fall back down to idle again. If you raise the revs slowly then you can hear it almost click in this booming/vibration as if something starts to operate/change operation at this rpm. The engine also does this when the car is being driven, in 2nd, 3rd, 4th, 5th & 6th gears. It sounds harsher when you are accelerating hard. Its a very noticeable and annoying booming/vibrating. I am really not sure what it is as I am not an expert on TDi engines, and am a little concerned what it is and is it serious? Has anyone had any experience of this or heard of this before.
Was also wondering what sort of rpm the turbo kicks in at I can hear mine start to make a slight whistling noise at approx 1300rpm, is this normal?

Any info gratefully received, thank you.

You may have better luck getting your question answered in the proper forum, as this one is for pics/specs. Maybe try the model specific forum for your Bora or the Performance section.