A5 2018 Thumping low speeds and turning, gearbox rattle

[zyx257]

Hi guys, 

I have just bought a 2018 A5 2.0L Petrol Quattro (252ps) with 60k miles from a private seller (First Audi) Basic visual inspection, test drive were fine and it had a full Audi service history so was reassured. 

Shortly after sale I noticed a deep thumping from underneath the driver/passenger seats/ possibly a bit further back. It seems to happen in high torque situations, e.g. at low speed uphill, while gently applying the throttle. Specifically immediately after a gentle throttle pulse, I'll get a thump. Then when accelerating off, it will happen at a slightly higher frequency 'thump thump thump' as I'm pulling away...you can feel it more than you can hear it. It's irregular, not a rhythmic beating but more random.

Furthermore when steering is locked and I accelerate, it happes at a very high frequency and becomes more like a rapid thumping that blends into a rumble.  

I took the car to my local mechanic who said no issues, just a loose undertray. 

Then took it to an indie who has experience with Audis and he found the rear transfer case of the gearbox is rattling badly - and thinks this is failing mechanically (videos attached of the rattle, more obvious on short clip). He also thinks replacing the gearbox will resolve the issue with the thumping.

I was initially wondering whether it was a propshaft bearing issue from reading other forums. It was difficult to recreate the thumping noise with the car on the ramp and the wheels floating. 

Obviously the gearbox / rear transfer case does seem to have an issue and it's looking like a costly repair. Just wondering if anyone had any thoughts as to whether this is a complete replacement job, and whether this might be causing that deep thumping noise as well or is that possibly something else that will need doing? 

Regretting slightly the private sale at the moment. Shame as the car has been a beaut to drive until now. 

Thanks in advance