2006 Volvo s40 2.4i

I replaced my HG the other day (I've done a few before with no problems). I was very careful marking everything. Locking the cams together with a cam locking tool specific for Volvo. And so on and so on. I put it all back together and everything went smoothly and aligned perfectly. Though I did not check for interference by turning the crank by hand.

I turned it on and LOUD "KNOCK KNOCK KNOCK.." maybe at a speed of 4 knocks/sec. I turned the car off. And noticed the engine was smoking from the exhaust manifold side. The only thought I had was "I screwed up the timing and hit the valves! Nooooooooooo!"

So to try and diagnose after looking around the car and finding nothing I decided to start the car again and quickly try to find out where the noise was coming from. I started the car no problem and when I got to the engine smoke started coming out again so I turned it off.

I took out spark plugs and turned the crank by hand and it doesn't seem to have any added resistance anywhere in the cycle. I stuck an inspection camera down in the spark plug holes (not the combustion chamber as it didn't fit past the spark plug hole) but didn't see anything unusual. I stuck a magnetic pickup tool in the combustion chamber to try and see if I maybe dropped a tool in there without realizing, but nope. Also I am assuming if my valves were hit and bent then my car would probably have died and refused to turn on the next time. Not sure if this is correct but a car with no compression can't run. I guess I should check my compression, but I'm just afraid of turning my car over

So my thinking now is that it is a bad exhaust manifold leak. I've never heard knocking like that before and I really doubt it is rod knock. The car was running pretty good before I noticed the HG problem. The HG never got too bad, I caught it pretty early. Compression before the HG change was 150 in all cylinders except for the 5th which was at 120.

The smoke I'm assuming is coming out of the leak. But that knock is the only thing I feel like I'm forcing to fit the symptoms. It's loud and is knocking not ticking IMO.

Can an exhaust leak cause a loud knocking?

  • What exactly is your question? Dec 3, 2015 at 21:23
  • Sorry I was focused on providing as much info as I can. Can an exhaust leak cause a loud knocking sound?
    – John_911
    Dec 3, 2015 at 22:31
  • Yes. The 4th Gen LT1 Camaro is known for it ... exhaust gasket leak sounds just like a rod knock. I don't know if your Volvo might do the same, but it is at least possible. Dec 3, 2015 at 22:39
  • Alright, well I am in the process of inspecting it. I doubt I'll be able to tell with just an inspection camera, but I'll try and tighten down the bolts a little more. Any tips or ideas other than tightening it down more? It has a new gasket on it already
    – John_911
    Dec 3, 2015 at 22:53
  • Instead of your inspection camera, try an automotive stethoscope. If you are in the US, you can pick one up from Harbor Freight for under $5. Use the tip and pinpoint where the noise is coming from. This will help you diagnose. Dec 3, 2015 at 23:12

1 Answer 1


It was indeed an exhaust leak. One of the exhaust gaskets wasn't on one of the studs so it didn't seal properly. I fixed it and the knocking is gone.

But it didn't get rid of the smoke completely, which is now confirmed as exhaust gases. I hope the manifold isn't warped.

  • You may have some residual smoke from doing the work. Give the engine a chance to completely burn anything off before you decide your engine is kaput. Recheck your compression to see if anything was affected. Dec 4, 2015 at 15:55

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