I have a 2013 Q3 2.0 Auto, which has recently started showing an unusual issue.

At some point within the first 20 mins of a journey from cold, a single brief metallic noise can be heard.
It only happens once, lasts for a fraction of a second and then won't happen again on the same journey.
I think its coming from the transmission tunnel but it is so brief that it is hard to identify its source.

It sounds like a very quiet, very brief, incorrectly selected gear.

When it happens the car is always in 5th gear at around 35-40mph and not under any load (ie usually just cruising in traffic, and not going up or downhill).

Overall operation is fine though. Gearshifts are smooth and there are no issues being seen with the operation of the gearbox.

Any ideas?