Pardon my ignorance of the 4P system but I could not find a schematic to reference.   I assume you have only the one outflow valve and not the secondary.   Taping over the outflow valve and not seeing a dramatic change in cabin differential suggests leakage elsewhere.  In the twins we would reverse a vacuum cleaner into the cabin through a makeshift connection and pressurize the aircraft.  It takes a while to fully fill the cabin but you get a good differential from a shop-vac or other device.   Of course, it is very dangerous to do this and such a connection must be done with all the safety precautions to prevent decompression or other problems while on the ground such as opening a pressurized vessel.   However, it was revealing where the whistles came from and perhaps this type of test could help isolate where the problem(s) lies.   From this thread I am assuming the turbo is operating and the slope controller is working too.  It sounds like those components are working correctly.   I would have first suspected the outflow valve not fully closing but you covered it up and took it out of the loop.  

Paul
N357V L2K
 
On 2010-02-03, at 7:42 AM, Jeffrey Liegner, MD wrote:

> I have flown with the Duke's Valve completely occluded via duct tape fully across the rear seat opening (for several month, in fact, part fo an experiment).  I was ready to puncture the tape if needed.  The cabin did not over pressurize, but it would have above ~13,000' to ~15,000' during power climb.  The initial slam of higher pressure during power up and take off was noticible, but not in excess of 3psi (it was just immediately noted).  Over pressurization at anytime was avoided with a controlled reduction of the throttle (MAP) to demonstrate cabin pressure correlation to MAP.  Upon landing, with outflow still occluded, cabin was always at airport ambient pressure before door seal was dropped.  Again, this zero differential is notable on short final, door seal tight, Duke's outlet occluded, engine below 11.5".
>
> During flight, outlet occluded, a small puncture through the occluding tape demonstrated brisk exiting air through the puncture out the Duke's Valve, but without a rapid cabin decompression (like one might experience with door seal deflation).  At any altitude, door seal deflation results in immediate cabin decompression and cabin climb to current flying altitude.
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> I also have a cabin alert above 10,000' and panel light notification.
>
> Jeff L