Transmission problem when coasting

I'm new to this, here goes. I have a 2004 xc70 that run through the gears great when I'm on the throttle, but when I leave off there is a vibration noise coming from the front of the car and the transmission seems to go into neutral or free wheels ,once I apply throttle it goes away and drive fine. If I do put it neutral as I am coasting,the noise becomes a higher pitch. I have only had the car since February and it run great although with no AWD. The angle gear was replace at one point but I believe the coupler is bad. Thanks for your help!!