Took the Audi assist technician out for a short drive and, annoyingly, the very loud grinding/knocking noise had disappeared. He put it down to a stone being lodged somewhere it shouldn't be.
I had two error codes for incorrect purge flow on the EGR valve (P044100) which he tested further using ODIS. The test came back OK so he just cleared that fault since I assume that's what caused the start/stop malfunction that I saw on the dash. He told me to monitor and give them a call if it happens again.
I do find it odd that an EGR valve fault wouldn't throw a check engine light and instead just cause a start/stop malfunction warning though?
There was also a fault logged (P04A100) for "Exhaust pressure control valve (B) Low" and he could not clear this one as it was static. I didn't press him to investigate this further as I have valve delete plugs in and that's probably the cause of it. Weird that it shows as a fault on ODIS, but not on any OBD applications like Carista and Car Scanner Pro?
@Phil3 Just a heads up about the above as I know you have the same valve delete plugs as me. ODIS still seems to be able to see the fault code.