The gurlgle or "grrt - grrt" noise in top gear around 1500 rpm is common here. I get it sometimes when not in tow mode, and always in Tow mode towing our trailer.
I recall from the other threads on this it was metal shield vibrating on the exhaust or manifold or something, can't remember
someone will correct me...anyway not a serious problem, just annoying.
Can't say on the SERV 4x4 message, I got it once last winter on a really cold morning, never happend again, and the dealer said there was no codes showing. wonder what this software update you got fixed?
And welcome to the group