Whenever I decelerate, I notice a rattle on front left side.
If I shift from drive to reverse, I notice a distinct “clunk” sound that never existed before.
If I put the car in reverse, I hear a loud tick/clunk each full revolution of the wheels.
When accelerating or maintaining speed, there are no symptoms, but going slowly or decelerating causes them to return.
Most likely a defective CV joint. If you have solid wheel disks, also check for pebbles inside the wheel disk.
Could also be a bad motor mount
Would a bad motor mount cause the clunking in reverse?
It could, yes. When you shift, there’s some torque applied that, if the motor isn’t mounted properly, causes the whole motor to shift. You feel that as a clunk.
Let me clarify that a bit.
There are two scenarios:
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Shifting back and forth between drive and reverse I notice a single clunk each time I shift.
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If I put the car in reverse, and backup for 100’ or so, it clunks each revolution of the wheel.
Are there any specific tests I could do to check which it is?
Just trying to make sure I’m chasing the right thing here 
So I pulled the left side drive shaft, and it’s OK.
No pebbles either.
I went to check on the right drive shaft, and I noticed facepalm, a hammer had fallen behind the engine. It was right next to the drive shaft.
At least it was a cheap fix.