Knock Sensor:

The car is at 89K, and started making some knocking noise (at least I think it is) driving uphill in reverse - at just enough rpm to move the car up.
I am not sure if it is related to the VAG code below, but does anyone know if "signal too small" is potential red flag?
The closest sound I can match is 14s-16s of below video - I found it very odd that the sound resembles an American muscle idle.
- the wop wop wop or thud thud thud idling sound before the driver presses on gas: https://www.youtube.com/watch?v=HptDoXO7kmc

A dealer quoted 1,340 for replacement over the phone, and I decided to either do it myself (if easy enough) or buy the part, and take it to an independent mechanic.
Apparently, there are 3 of them, front left/right, and rear left, but I could not figure out which one would be G198 sensor 3 based on my search.


Engine Mount:

After getting the engine mount code, I took it to a independent mechanic specialized in VW/Audi, and they are quoting 250 for the labor for replacing one engine mount.
These guys have good reputation, but not sure if 250 per mount replacement is good price or not.
Is it something that a normal shop can handle?

17729 - Knock Sensor 3 (G198)
P1321 - 002 - Signal too Small
Freeze Frame:
Fault Status: 01100010
Fault Priority: 0
Fault Frequency: 1
Mileage: 140203 km
Time Indication: 0
Date: 2016.04.23
Time: 11:27:11

Freeze Frame:
RPM: 2806 /min
Load: 90.9 %
Speed: 61.0 km/h
Temperature: 93.0∞C
Temperature: 31.0∞C
Absolute Pres.: 1000.0 mbar
Voltage: 14.097 V		 17981 - Left Engine Mount Solenoid Valve (N144)
P1573 - 004 - Open Circuit
Freeze Frame:
Fault Status: 01100100
Fault Priority: 0
Fault Frequency: 1
Mileage: 140201 km
Time Indication: 0
Date: 2016.04.23
Time: 11:21:23

Freeze Frame:
RPM: 689 /min
Load: 44.7 %
Speed: 0.0 km/h
Temperature: 90.0∞C
Temperature: 53.0∞C
Absolute Pres.: 1000.0 mbar
Voltage: 11.303 V

Thank you so much for your help,