My T3 R-Design MY22 makes EXACTLY the same noise. Rattle or tapping very obvious accelerating between 2000 and 4000 rpm but no noise idling or cruising. Car performace is fine. I have also been told by the Volvo dealer that it is normal for the 3-cyl T3 engine. It is very hard to get used to as is sounds instinctively abnormal! An indy said I was lucky there was only one noise from the engine!

 

As the T3 engine is so weak it will sound like a diesel until the turbo spins up and can help the machine really move. It is a combination of the automatic gearbox configuration and the need of the turbo needing to spin up to start moving the vehicle to more "normal" speeds.
Sorry to say you will have to get used to this. The T3 is just too underpowered and the gearbox seems to assume you want to travel at speeds lower than 30 mph. I seriously think the T3 was created for intra-urban driving only and not for driving on interstates etc...