Sounds like a boost leak but no boost leak..

[newsh]

This is my mate's B5 RS4. As you accellerate and the turbos spool up there's a squeesh noise, sounds like a boost leak. We've put bungs in and it will hold 1 bar for a while, dropping to .5 over say 30 secs. It's leaking down through the valves and rings and breathes gently out through the PCV valve. There's nothing that sounds like a leak.

There is a noise like a small exhaust leak coming from below the car, could this be it? I'm wondering if the EM to turbo joint is leaking on one side or the turbo to DP? Would this make a squeeshy sound? Need to get this sorted so PRJ can map it and I'm on holidays next week.

We've just sorted the valve timing and replaced all the valves in the PCV system and most of the hoses and it's flying on it's new hybrid turbos but now this new noise

[UrS4boy]

What about a ripped diaphragm on the BPVs (2 BPVs IIRC)??

[mike74]

Maybe the dump valve(s) leaking under boost? Try blocking off the pipes they connect to and go for a spin to see if the noise is still there

[newsh]

Tried vacuum with a syringe on the DVs. Seem ok, but could block them and test, thanks.

I spose the diaphragm could be ok but the valve stuck open with gunk..

[newsh]

More info, the car is not boosting properly at all, when the boost gauge goes above zero it sort of pulses and feels like a mild misfire, makes a max of about 10 PSI while pulsing and only when above 4k rpm (I'm told).
Exhaust sounds weird, like it's running on one bank.
No leaks, no codes, no misfires logged. I drove it briefly up the road at low-ish rpm, it feels really gutless and doesn't boost on a small throttle opening, boost will pulse between 0 and 5 psi if you hold the throttle half down.

I'm thinking it may have a collapsed cat

Oh yeah, the squeesh noise was the exhaust leaking from where the cat joins the centre pipe. Both of these joiner tubes started leaking..

[Bowie69]

Take the pre-cat lambdas out and go for a drive, if it livens up then it is the cats, but yes it does sound like that.

[newsh]

Take the Lambdas out so the exhaust leaks? To let the pressure escape?

[K Simmonds]

It does sound like a cat has collapsed, I take it there's one for each side of the the V?

[Bowie69]

Take the Lambdas out so the exhaust leaks? To let the pressure escape?

Exactly, a collapsed cat would just stall the turbo hence no boost, I'm sure you would agree? No lambda means the exhaust breathes, albeit rather loudly

Good temporary test at least.