For a few months now I notice that between 3 and 4k rpm my car does a tssssssssss sound which comes from the turbos.
You really need to listen carefully to hear it, I did so many datalogs which look good to my tuner.
I then changed spark plugs but still the same.
I tried running it on race fuel and still the same. I'm pretty sure that the sound comes from the turbos VTG mechanism moving up and down like crazy between those rpms.

Anyone who knows why this happens ?

 

Did you do the VTG lubrication service a couple years back? A notice went out, I think some left the factory without the proper lube.

Also check the diverter valves perhaps?

 

Does it happen under full throttle or when you are shifting.

If it happens between shifts it is the wastegates releasing boost pressure & that is normal.

 

Maybe your BRV's are blown\torn, you on OEM or aftermarket BRV's?

 

How do you know you're losing boost? Durametric? Last log you posted looked really great. No boost loss at wot if I remember correctly.

 

BRVs are after Market and were checked. I hear the tssssssssss sound between 3 to 4k rpm, it is more noticeable when shifting to 3rd/4th at low rpm 2000rpm and full throttle to red line.

Dude, I don't think I loose boost but it's like the VTGs retard. It was not doing this before.

I don't think is the BRVs because the problem happens under specific rpm

 

Switch back to stock BRV's and see if it goes away.
Todd asked me to switch to stock when he started working with me on my car.
Quite a few vendors\tuners recommend stock BRV's over aftermarket.