Knocking under load on first gear

I have this issue.. I already asked on FordOwnersClub, but I feel like here I might have a better chance to sort it out.

I bought a Ford Focus mk3 2011 with 1.6 ecoboost 134kW engine with 90k km. My issue is, that under heavy load (heavy acceleration or acceleration on a steep hill,) I can hear a knocking sound. It does only on first gear and only under acceleration. First I thought that it could be comming from a driveshaft, because I also felt vibrations on highways and I had that issue with my mk2 earlier, but it turned out to be a different problem (wheels not balanced.) Suspension seems to be okay, but I had a coil spring broken, which I had to replace.

So I started digging and observing and I thought that it might be LSPI, but I already had 100 octane petrol in it (europe). Basically best you can get here on gas stations. Anyways, I tried a different brand with similliar petrol quality and it stopped doing. Well, it had to be the petrol I put in. Turns out it wasn’t. On the same tank 500km later, it started doing again.

I also recorded the sound and inspected it with spectrogram. Here is the audio https://sndup.net/3zt2. It starts at 4s.

Things that I noticed:

• It happens only on first gear under load with good petrol quality and even additional additives
• It happens around 2500-3000RPM
• It does not "knock" immediately after I press gas pedal. and even in the recording or the spectrogram above it is apparent, that the revs first built up to around 2500RPM and then it started knocking – it does not start knocking after I press gas pedal immediately
• You can hear multiple "bangs" in one "knock". It is most apparent in the second knock. The first "bang" is quiet and not much noticeable. You can also see it in the spectrogram.
• I only tried it with warm engine
• When it starts knocking, I don’t feel any power loss or any output in the steering wheel
• You can also hear that the revs are building up more rapidly when (after??) it starts knocking. It might be a coincidence or it might be the turbo kicking in. Although, I don’t think it is a turbo problem, because otherwise it would be noticeable in other gears as well. Especially on highway.
• The intensity of knocking varies. It can be loud (just like in the recording) or I can’t hear anything. Even on the same tank.

After I thought long about it, It does not sound like a premature detonation. It is a metallic noise and even the frequency does not indicate the LSPI. It would have to be much more frequent. The weird part of it is the stable frequency of the knocks.

The problem is, that it only does it on the first gear. I would say, that it is a transmission problem, but it can also be caused by the bigger stress on the drivetrain on lower gear.

I will be very glad for any kind of advice. I don’t want to spend thousands on changing parts randomly hoping it would solve the problem. I would rather at least pin down the problem to engine/turbo/suspension/diff/trans/clutch or anything else it might be. One guy said that it might be bad suspension and that the sound might come from the wheel "bouncing". I certainly think it is a possibility. I only recently changed the coil spring, because the last one broke so that might be a clue, because coils don’t usually break at 80k km.

Any help will be much appreciated.

I used these tools to inspect the recording

Spectrum Analyzer | Academo.org – Free, interactive, education. – spectrogram
Free Online Time Stretcher | OnlineToneGenerator.com – to slow the recording