Little noises 2011 Automatic

[1st time ford]

High everyone, this is my first post here. I had a 2011 with a blown motor popped in my lap, so i had a mech do a motor swap. Starts , runs , shifts well , but i hear a whirling sound on deceleration. I am not sure its origin ... checked transmission fliud (running and in neutral) via fill plug and is OK ... catalytic converts r a little plugged due to previous motor head gasket added coolant to exhaust ...

Any ideas on the noise is hear ... is if normal to hear a noise decelerating ?