2003 Avensis 1.8 Exhaust Manifold

[Farmerpug]

I recently bought a 2003 1.8 Avensis T4 with 140,000 miles.

It seems generally good but on the road going past 4000 rpm, it emits a loud whistle/chirping noise. It's best described as sounding like a dump valve, although this car is very much original.

I replaced the gasket between the manifold and main length of exhaust pipe, the noise still remains.

Are cracked manifolds ever a problem on these, I have removed the upper heat shield and can't see or feel any obvious exhaust leaks. At idle it is harder the induce this same noise, it comes in closer to the cars rev limiter.

Just wondering if the manifold is worth removing as the rusty bolts in the area especially on the heat shield seem to be keen on snapping. 

[Konrad C]

I have done manifolds on different cars, most recently changing a friends 2005 Smart ForFour Brabus (177 hp). If you are going to remove the nut/bolts/studs, stray plenty of plus gas, WD40 or similar at least a couple of hours before. That's what I did on my old Avensis, spray the night before. Good quality sockets and spanners that minimize rounding of the nuts help too. Looking for a crack can be difficult. The crack could be on the underside. You need to get the car up on a lift or use a mirror to inspect the whole manifold. The car need to be under load, with a wide open throttle to really hear the noise, I suspect. 
Another source of noise can come from the induction side. Check the throttle body and inlet manifold. Also check the vacuum system, and the engine(PCV) vent pipes.  

[Konrad C]

I just noticed you have posted on Facebook too. When changing any gasket, the mating surfaces need to be clean. But if the noise still happens, check the exhaust thoroughly. Still check other possibilities I mentioned above. 

[Farmerpug]

Someone has mentioned the catalytic converter may have broken up.

Incidentally i did get the car with the engine management light on and my fault code reader said catalyst below threshold efficiency (something to that effect i am paraphrasing from memory)

Would that explain a lack of external leak?

[Farmerpug]

I got the manifold off but not out of the car, some issues getting it out of the gap between the engine and the bulkhead.

However i don't see any catalytic converter within the manifold, and some sources indicate there is one inside the manifold.

Is there any way to get parts diagrams for the car using its VIN to find out what type of manifold or catalytic converter it should have?