GTI experts? Car has loss of power and horrible noise at high revs

[Rossmk5gti]

My car has the following symptoms and no codes:

Loss of power, still boosts however power goes dead at 4000rpm plus

Noise from the engine bay which gets louder as revs increase, sounds like a diesel being revved hard

Whilsting noise that sounds like a siren occasional (usually on startup)

I have brought the G revision of the diverter valve however im concerned about the horrible rattle when the car is revved hard, low down in the rev range the car performs fine with maybe a slight power loss.

Any help would be appreciated I am concerned the turbo may be on it's way out or the engine may have moved out of time. 

[ady-gti]

Exhaust cam chain possibly.. It will be worth changing the DV to see if it sorts the whistling, but the high end noise etc could well be the chain has stretched too much and skipped a tooth.

[Chesto18]

Any engine management lights?

It sounds like you’re describing a mechanical sound, which I’d guess at a cam chain issue.
Could equally be a misfire if it’s holding back above 4K and you’re having a flashing engine light. Wouldn’t sound as you describe though.

Could be a turbo issue too, as it spools up with the increasing revs.

Doesn’t sound like the DV though.

[Rossmk5gti]

The more I drive it the more I think it's a turbo issue.

There are no lights on currently, I've noticed though the car has boost when I first start it up but the longer it's driven the flatter the power gets.

[ady-gti]

Did you change the DV? If you did was it split? I think you have a smallish split in the DV that, when cold, is managing to maintain roughly the boost required. Then, when hot, the rubber gets more malleable and therefor lets through more air. Even if you've checked the DV and thought that it's fine don't put it back in, just replace it and see if it's cured - eventually you'll need to replace it anyway as they are a weak point.

I changed mine to a revision D DV, the metal one that was mainly used on audi at the time I believe, as it didn't have the wide rubber diaphragm that is/was susceptible to ripping.

These engines are noisy, for mine it's mainly the injectors, but if it sound like a diesel then it's possible the cam chain has stretched and needs replacing at some point.