SEman

I have a 64 reg 170PS DSG 4x4 which has done 45000 miles. I bought it in March 2017 with 25000 miles on the clock, as the second owner and it has a full service history. I tow a 1300kg caravan and the car has performed very well. At its last service in February 2019 the diagnostics picked ups problem with the Haldex. Further diagnosis showed the haldex pump to be faulty. A new pump was fitted in April 2019 and the haldex fluid renewed. It has had an earlier Haldex fluid change whilst I have had it. Since the car has had the new haldex pump I can't say that i have noticed any significant difference in its sol driving characteristics. However last week when I towed the caravan since the pump had been replaced I noticed a thrumming vibration but only under certain driving conditions. If I was doing 50 mph on the flat and accelerated to 60mph then i felt a vibration cut in which ceased when the power demand on the car reduced. Similarly when coming to a slope say at 55-60mph the thrumming vibration would start but stop once the slope eased off. The car had the load equivalent of four 75kg adult inclusive of myself, wife, dogs and luggage. The caravan noseweight was 70kg include in the loaded weight. Tyre pressures were 280kpi rears, and 240kpi fronts. Tyre size 225/45/17 Michelin Cross Climates. Yesterday taking the caravan back to storage I accelerated from 30mph to 50 mph on a flat A road and no thrumming vibration. Tyre pressures 220kpa front 225 spa rear and with car unloaded. The previous services had been in September 2017 and June 2018 and neither showed any fault on the diagnostics and when towing the car was just as quiet solo as when towing apart from some increase in engine noise under towing load accelerating or on hills. Certainly the 4 wd system was working okay in September 2017 as I had to exit wet grass field with the caravan in tow and it all worked fine. I have taken the car to the garage that replaced the haldex pump and they carried out a diagnostic and it came back okay. Any thoughts on what might be causing the thrumming vibration would be welcomed.