ovelaur

I do not where to start on this issue, so hope someone in the forum at least have some suggestions where to start troubleshooting. As this is an intermittent fault, I would rather do some troubleshooting myself as the Audi dealer is a little bit far away from my place.

The car:
2012 Audi A6 Avant Quattro 3.0 TDi Biturbo, 55000km

The problem:
Sometimes, and sudden, the low frequency noise from the exhaust increases significantly. This is most noticeable between 1000 and 2000 rpm and with a constant and low and steady throttle. When accelerating very slowly, by intention, the rpm increase is oscillating with a very low frequency (period around 0.5-1 seconds). The problem disappears as sudden as it arrive, sometimes during a drive and sometimes after a full stop.

What has been done so far:
Using VAG-COM VCDS to read any fault codes as EGR valve was suspected to have an intermittent fault. But there is no fault codes present or stored.

What values should I log, for example to graphs, in VCDS to continue troubleshooting? My problem is that I am rarely in a position to do any logging when the fault arrives. Is there a storage memory in the ECU where such values are logged over time that can be downloaded with VCDS?

Any suggestions are appreciated.