Help identifying a noise problem?

I have a 2003 Toyota Corolla with approx. 98k miles.

Recently I've been changing out plenty of parts as I know the previous owner neglected to do any maintenance. Recently after replacing the front and rear motor mount along with the transmission mount, there has been a clunking/grinding noise. It ONLY occurs when the car is decelerating below speeds of 30. Its really noticeable when reversing up a hill especially if I turn my wheel slightly left.

I've done this so far to see if its the cause of such noise;
- Driver side axle
- Driver and Passenger Side Sway Bars
- Front Motor Mounts
- Rear Motor Mounts
- Transmission Motor Mount

I've had my neighbor (who's a mechanic) check other parts... he said all looks good besides struts needs changing soon. Called my grandpa (whos also a mechanic), he noted that it might be the transmission.

So my other concern is... if it is the transmission... is there any way to remedy the situation without having to replace the entire transmission? I will be doing a transmission fluid, filter, etc. Sometime this week.

Any help or suggestions is much appreciated.