Tried using the search but got nothing conclusive, so hoped someone could help....

Mt octavia, PD140. Making the police siren noise so took it to my usual mechanic, who seems to think its a boost leak somewhere, but as its on warranty I want to make sure before its too late!

Symptoms as follows:

MPG and power delivery seem fine. Though its almost as if theres a little "kick" when you come off the throttle.
Noise is only heard 1000-1800rpm.
It doesn't seem to coincide 100% with turbo speed though, it will slowly raise if I'm using gentle throttle, but as i come off the throttle it will give a quick rise in pitch for about 1 second then drop to where it should be (if you can call it that!)
If I keep at constant throttle/revs it does seem to fluctuate a little. Sometimes the noise appears when at idle, other times not.
Which makes me think its more a slight leak........but then surely Power would be affected?
It runs perfectly fine up to the redline. So not an obvious turbo fault, but highly irritating when driving!

Is there any way to confirm what it is? Would a VCDS scan show anything up? Got around 21 days left on warranty so would need to know soon-ish.

Turbo wear is usually a siren / whistle, a leak is usually a whoosh noise.

 

The symptoms you describe does sound like a leak, if it's only a small leak then your unlikely to notice a drop in power.

 

If boost was effected enough by the leak then you'd notice a drop in power and likely see the EML or limp-home mode. It would be worth scanning the car for fault codes though. 

Definitely a siren noise! It does have a bit of a stutter around the 1700rpm mark at certain pedal positions.
Also noticed today that the mpg reading is way off, by about 20%! I can understand 5%, possibly 10%, but 20? Could this be down to it measuring vacuum pressure from the same place as the VNT system (Which could be messed up) causing the miscalculation?

Going to hopefully get it scanned with VCDS in the next few days.

the easiest way to find out whether it is a turbo is to get under the car remove the turbo intake pipe and visually inspect the compressor wheel for any sign of damage plus grab the shaft and move it side to side to see if the play is not excessive usually 0.5mm is acceptable on a second hand turbo. If you hear siren noise in most cases the compressor wheel is already rubbing the housing around it or the blades are damaged...

Thanks for the advice! If nothing shows up on the VCDS scan ill see if I can get my mechanic to take a look.
The noise seemed a bit louder today. Strange that it sometimes does it at idle and other times not. Wondering if expansion with heat is causing it to rub at idle after driving.

Interesting point on the MPG reading.

 

I went through three turbo's in as many months (don't ask - luckily under warranty!) on my 2007 PD170 vRS.

 

At the same time my average MPG readings went to pot, never to return to normality again!