Noise in the outdoor area with controlled residential ventilation - noise regulation regarding time?

As already mentioned, [ta Lärm] will probably settle the matter. Strictly speaking, 40db at night apply at the property boundary.

are strictly valid at the property boundary.

No. The location of the emission points is clearly defined in the TA Lärm. I cannot explain where this unfounded, blanket, persistently maintained gross nonsense about property boundaries comes from.

No. The location of the immissions points is clearly defined in the TA Lärm. I cannot explain where this unfounded, general, persistently spreading nonsense about property boundaries comes from.

well, that is not quite correct. under point 2.3 it says:

If within the impact area of the plant, due to pre-existing loads, it is to be expected that the immission limit values according to number 6 will be exceeded at another location due to the additional load, then the location at which the total load exceeds the relevant immission limit value according to number 6 the most must also be defined as an additional relevant immission point

this means: if the neighbor complains that the neighbor’s plant is too loud on their terrace, the measurement will be taken at the said location.

6.

d)
in general residential areas and small settlement areas
day/night 55 dB(A) / 40 dB(A)
e)
in purely residential areas
day / night 50 dB(A) / 35 dB(A)

We measured 67db (at night) directly at our garden fence on level 2. In terms of volume, in my opinion, this was comparable to the outdoor unit of an air conditioner and definitely too loud.

Well, I don’t know what kind of air conditioners you have seen so far, but obviously none of them were proper ones. No offense, just stating a fact.

My (industrial) air conditioner in the workshop produces a maximum of 47dB(A) at the outdoor unit.
The air conditioner I installed for my parents (high-quality consumer unit) is only 42dB(A).
For comparison: 45dB(A) prevail INSIDE a quiet(!) apartment.

If you stand outside (very quiet dead-end street, next busy road several hundred meters away) under my air conditioner, which is mounted at about 2.5m height, you have to look at the fan to find out if it’s running because the rustling of leaves in the wind is still louder.

Of course, if you buy one of those Frigoline China units, you have to live with the corresponding noise levels. Although even those nowadays comply with 56dB(A) at 1m distance. That still makes 42dB(A) at 5m distance.
So my personal opinion: If you hear a heat pump or air conditioner, something is broken or it’s cheap junk. Nowadays, it’s nearly noiseless.
Whether the installer who put in the unit (and has to rework it for free if something isn’t right...) is the right contact for an independent assessment is another matter. Doesn’t matter whether it’s controlled residential ventilation, air conditioning, or heat pump.

well, that's not quite right. under point 2.3 it says:

You are quoting very selectively. You have (deliberately?) omitted the section that defines what a relevant immissions location is and where it is referenced how its position is defined. Professionally and according to the TA Noise, the position of an immissions location is correctly defined according to the subsection you partially quoted as follows:

for built-up areas 0.5 m outside in front of the center of the open window of the noise-
most affected protected room according to DIN 4109, November 1989 edition

means: if the neighbor complains that the neighbor’s installation is too loud on his terrace, the measurement is taken at that location

Correct, that definitely makes sense. BUT: as soon as an attempt is made to derive from the measurement results of a measuring point with an arbitrary location the immission guideline values of TA Noise (which are tied to the aforementioned location of the immissions locations), you will fail. From the relatively simply determined measurement value at a measuring point, an assessment level must in any case be formed, which is then interpolated to an immissions location according to TA Noise. Only the assessment level at an immissions location may be compared with the immission guideline values of TA Noise! Measurement values or even assessment levels away from these fixed immissions locations on terraces or at property boundaries are completely irrelevant according to TA Noise.

I never said anything else either. And of course I quoted selectively, because the exceptions are listed there. And I never spoke of an arbitrary point. But if the noise bothers the neighbor at the boundary because that's where his sleeping place is, then the boundary is the reference point. Basically, any point can be the reference point if there is a (reasonable?) reason for it.

I myself studied acoustics for 2 semesters during my studies. I am familiar with the procedures and calculations for determining the sound pressure level. But no one cares about that here and it is presented in a strongly simplified way.

35 dBA is really little. Even small disturbances can already drown everything out...