Some of you may know that I’ve recently had to replace my cylinder head following a massive HG failure. I replaced the head with one which had been skimmed, and I had lapped all the valves and had it fully cleaned.
The car now seems to be running really well, with the exception of a slightly odd noise which I can’t identify. I don’t know if the noise was there before, as I know I’m now listening out for anything unusual so I’m more sensitive to its little quirks.
It’s only once the engine’s warmed up. Between 1500 and 2000rpm, there’s a slight ‘resonance’ coming from the cam cover area. Specifically it seems to be localised around the front cylinder, and coming from the very top end. It’s not a vibrating noise as such, or a grinding, just a kind of reverberation which sounds related to something spinning. I think it might just be something like the cams causing little pockets of air pressure under the cam cover at certain rpm, but I’ve never heard it before. Though I’ve never listened for it before.
Otherwise the engine is running perfectly, no flat spots and revving smoothly. Feels like a brand new engine in fact.
The other thing is that the exhaust downpipe is blowing very badly at the moment and will need something sorting out soon. It’s blowing from the flange to the cat. I wonder if what I’m hearing is some kind of resonance from the exhaust resonating through the head. It does seem like the exhaust only really blows badly when it’s warm, and it’s only when it’s warm that I can hear this sound.
Anyone got any thoughts? Ever heard anything that matches the description?
I decatted mine and it sounds much more tinny and reverberates quite loudly, seemingly from around the downpipe to cat (sounds like it’s coming from the passenger footwell when driving), and more noticeable when warmed up and around the revs you mention and on a gentle throttle.
Perhaps the exhaust leak is resulting in an airflow similar to a decat pipe?
I’d fix the exhaust first and see if the noise changes, if not at least you can eliminate it as a possible cause.
Also, check that the bolts on the exhaust manifold cover are still tight - one of mine had loosened right off after about 500 miles after the rebuild allowing the shield to rattle around.
Thanks Jim
The noise itself isn’t coming from the exhaust or the manifold, it’s definitely coming from the head. Specifically it sounds like it’s coming from the area of the cam lobes for the #1 cylinder. It’s kind of like the noise you would get if you blow across the top of a bottle, but a bit more muffled and rhythmic. I’m just wondering if the blowing exhaust is creating some kind of pulse resonance or something, weird as that might sound. I can’t really think of anything else that might make a sound like that.
Maybe the blow by is causing a harmonic to be set up somewhere in the head. I would fix the leak before worrying too much about strange noises. If it’s still there than we all need to think again 
Ok, so the idea is not complete nonsense then at least.
It’s been bad ever since I changed my decat for a cat from MX5Parts. The cat always seemed like a bad fit, didn’t seem to want to seal properly (took loads of gum as well as the gaskets) and once I’d got it on, the exhaust had been shifted to a position where the rear section was hitting the subframe under hard cornering. Obviously now it’s started blowing again.
I’m going to change back to to the decat this weekend and see if it improves things at all, as the decat is a perfect fit and never blew in the slightest.
I used loads of that red high-temperature silicone sealer on mine as it’s flexible and seemed to survive very well on the exhaust pipe on my kit car. Got it from Screwfix IIRC.