Cracked exhaust manifold?

[SteveSpeed]

I have a strange howl/whistle from my engine. It comes in around 2,500rpm and comes and goes, sounds like induction or exhaust. The dealer I recently bought the car from changed the belts and the aux tensioner, which is what we thought it was, but it's still there. Comes in when engine is hot, then comes and goes. I have a feeling it may be a cracked exhaust manifold, as it is affected by a) heat and b) trailing to balanced throttle only (ie, when the exhaust will have some back pressure). Floor the throttle and it goes away.

Makes it sound like a poorly setup turbo!

Otherwise car/engine is perfect, only 18,000m, engine sings and revs and produces good power.

Have you heard of this before? I read in a buyers guide that the manifolds can crack.

Any ideas, verification or not of my diagnosis is appreciated.

 

[Raptor6767]

I don't believe that it could be a cracked manifold if the car has low mileage, it might be the gearbox sound that the car can produce at certain rpm.

 

[Fernandez]

Not sure either but I dont think its a cracked manifold. I see from your other post you are around the Sheffield/Chesterfield area. I'm near Chesterfield so if you want to compare engine noises give me a shout

 

[Foxspeed]

if its exhaust manifold it will blow and sound "rough" - probably depending on how and where its cracked it will get noisier when hot....

could also be a leak near the cat underneath

 

[SteveSpeed]

Thanks for your comments, they're helping me think it thru. It's more of a whistle than a blow, so now thinking induction, it's a bit like an old fashioned kettle. It's definitely not gearbox, as I can replicate it when hot at standstill in neutral around 2500rpm, so not anything linked to transmission.

It's very strange as it constantly changes pitch as you feather the throttle and can go away one second, then come back, are there any valves in the induction system that could be sticking?

I am going to check the air box, pipes etc... For good fit.

I also wondered if the previous owner had changed/removed something in the induction system, is anyone aware of anything obvious?

 

[Fernandez]

Might sound silly but is the lid on the airbox fixed down properly?

 

[SteveSpeed]

Update, just had the car overheat whilst ticking over after a short drive, checked the engine bay fusebox and the 40amp fuse on the left was disconnected. Reconnected the fuse and went for a local runaround, the noise has gone and the overheating has too!

Anyone know what the 40amp fuse protects? There is nothing in the owners manual.

 

[Foxspeed]

Update, just had the car overheat whilst ticking over after a short drive, checked the engine bay fusebox and the 40amp fuse on the left was disconnected. Reconnected the fuse and went for a local runaround, the noise has gone and the overheating has too!

Anyone know what the 40amp fuse protects? There is nothing in the owners manual.

engine cooling fan?