Conudrum - evidence of rodent nesting within inlet manifold overnight (nuts aplenty and dead mouse), rattle in exhaust (heavily clad bend just prior to horizontal run (sorry do not know technical term for this area of the exhaust) and significant loss of power accompanied by loud hissing from inlet mainfold. Only really concerned about latter character of otherwise superb car (apart from petrol consumption - painful in UK and France). No evidence of damage to spagetti of pipework and electics surrounding the manifold area but loss of power coincident with hissing is significant (intermittant but only occurs after car has been running for 20 mins and more prevalent with heavy right foot). Enough rambling - anybody had similar problems??? BT

My guess, and it's just a guess, is that your rodent findings are coincidental to your hiss, which sounds like an intake manifold leak. Try squirting soapy water around connecting points of the intake manifold when you hear the hissing. Listen for changes in the sounds of the hissing and the engine and look for bubbles or places where the soapy water is sucked in. Not sure about the rattling sound, but it could be unrelated to the hissing and just some of the shroud rattling. Good luck!

Could your wee furry friends have chomped on some of the vacuum lines? Might be worth a really close inspection of the rubber tubing and connections.

Another thought based on KStretch55's approach--with the engine idling, spray "starter fluid" around the vacuum hose connections. Then if there's a leak you should hear a slight revving. Never done it but it sounds like a neat trick!

 

And I tend to agree, the rattling is probably loose heat shield/shrouding--a common problem. Do a search here on the board as it's been discussed recently. Good luck!

My thoughts exactly although the hissing is such that the engine sounds like a steam engine, it only cuts in after 20 mins or so and all the hoses look sound. It seems to be some kind or resonance with one of the control valves.

My guess, and it's just a guess, is that your rodent findings are coincidental to your hiss, which sounds like an intake manifold leak. Try squirting soapy water around connecting points of the intake manifold when you hear the hissing. Listen for changes in the sounds of the hissing and the engine and look for bubbles or places where the soapy water is sucked in. Not sure about the rattling sound, but it could be unrelated to the hissing and just some of the shroud rattling. Good luck!

My thought was that perhaps once the engine is good and warm some component has expanded enough to shift and allow an opening or "crack". Another thing you can do that sometimes helps pinpoint such noises: take a length of hose (garden, fuel, heater, etc.) and put one end to your ear and carefully scan the engine with the other end. Usually, you'll be able to narrow your scan right down to the offending item. Hint: use a hose that's long enough for you to put one end to your ear and still be able to see the other end without kinking it. If you kink it the sound is blocked. If it's too short you can't see your hand and there's too many moving parts on a running engine to do that safely.

Conudrum - evidence of rodent nesting within inlet manifold overnight (nuts aplenty and dead mouse), rattle in exhaust (heavily clad bend just prior to horizontal run (sorry do not know technical term for this area of the exhaust) and significant loss of power accompanied by loud hissing from inlet mainfold. Only really concerned about latter character of otherwise superb car (apart from petrol consumption - painful in UK and France). No evidence of damage to spagetti of pipework and electics surrounding the manifold area but loss of power coincident with hissing is significant (intermittant but only occurs after car has been running for 20 mins and more prevalent with heavy right foot). Enough rambling - anybody had similar problems??? BT

 

When the hiss and power loss are occuring, is the exhaust gas coming out of the tailpipe with full force or does the exhaust gas flow from the tailpipe seem weak? Reason I ask? I suspect the catalytic convertor is restricted or the substrate (the "honeycomb" inside the convertor) has fallen apart. That would result in a loud hiss, especially upon acceleration, and a loss of power. Consider attaching a vacuum guage to one of the intake manifold vacuum ports, hold the throttle at a steady 2000 rpm and watch the guage---if the vacuum reading falls at steady speed, there's a restriction in the catalytic convertor. (Make this test with the engine fully warmed up.)