Tuned ASZ: Vibration Issues - ZMS or Engine Mounts?

Hello,

"When running at full throttle between 2000-2500 rpm, in the range of maximum torque (fuel injection amount = 65 mg), I often notice an additional humming frequency with a well-worn engine. It's not dramatically loud, but as a driver who pays close attention to the engine, it's noticeable – although it's not comparable to the clattering and knocking of a heavily modified AFN engine, which I thought was about to fall out of the car."

Since I have never noticed (fluctuating) vibrations as a mixed product with the engine speed during this additional hum, I suspect that the additional hum is a rotational subharmonic, meaning it corresponds to, for example, half the rotational speed.
As long as the additional humming occurs, the overall engine noise sounds somewhat louder and deeper.

I've been thinking about the cause for quite some time, but I'm always wavering between...
a) ZMS (at full capacity) and...
b) At least one engine mount in which the rubber component is deformed to such an extent by the increased support force that the metal parts on either side of the rubber come into contact, forming a "thud bridge" between the engine and the body.

What do you think or what are your guesses?

I'm guessing it's b) - it wouldn't be that uncommon in a 9N.

The ZMS (presumably referring to a specific component) will definitely be overloaded under full load and is at its limit. However, the engine mounts are also under greater stress because the engine is relying more on the chassis to absorb the torque. The defective mounts should be easily identifiable by, for example, spraying the rubber mounts with yellow paint and then driving a short distance. You will then see where the paint has been worn away, indicating where the mount is failing. This is similar to how crash test dummies are used.
I also drive an ASZ engine and am extremely satisfied, but I still want at least a chip tuning. Of course, only "Dieselschrauber tuning" is an option for me .

ulf wrote:

... What do you think or guess?

c) An exhaust system that transmits vibrations because its metal has become "soft" or "worn" over time.

It's not exactly common, but I experienced something similar with my 1Z. The resonance noise disappeared only after the exhaust system rusted and fell off. With the new exhaust system, the noise was gone as well.

Bertil wrote:

c) an exhaust that transmits vibrations because its metal has become "soft" over time...

Hmm... it's definitely torque-dependent: this buzzing never occurred with the stock configuration.

In addition, the buzzing sounds "digital" to me: it's either there or it's not.
I have never (consciously) observed a gradual increase or decrease in power (e.g., at full throttle and increasing RPM = a slight decrease in torque), but rather it disappears abruptly, as if it were switched off.
Was that also the case with your drone exhaust?

ulf wrote:

... Was that also the case with your drone exhaust?

Yes, it was load-dependent.
With the leash attached, it was noticeable; without it, it wasn't there. And when it was completely detached, it was either there or not. The whole thing sounded like a standing wave.

I never suspected the exhaust. I also don't necessarily think it's the same for you. That would be like hitting the jackpot.

About a year ago, I had a Golf here with a 110 horsepower engine that had been modified. It did something similar. Under load, it made a roaring noise, but no noise otherwise. I also suspected it might be the dual-mass flywheel or the clutch. The engine mounts were new.
After an unavoidable rear-end collision, the problem supposedly disappeared (as the owner later told me). Unfortunately, I don't know what was done during the repair. An exhaust system might have been involved.

Interesting. I'm observing a similar phenomenon in my Fiat 1.9 MultiJet 16V. Starting at around 190 km/h or 3500 RPM, there's a slight vibration that occurs every 0.5 seconds. I've suspected the flywheel for quite some time.