Ever since I got my front brakes and rotors replaced, I sometimes hear this knocking sound when I put it in reverse or drive (2wd automatic). It's only a single knock and doesn't necessarily happen when i turn the wheel. I have no fluid leaks. Any suggestions?

 

It could be the new brake pads moving back and forth in the caliper slots.
If the caliper has a lot of corrosion in the pad slots, it will be scraped away by the mechanic, leaving a slot larger than the pad. This gap will allow the pads to slide back and forth in the slot causing a single clunk.
I get a clunk when I back out of the driveway and hit the brakes. I distinctly remember chipping large rust flakes off my calipers in the pad slots.
You will have to figure a way to shim the pads or get new calipers, or maybe the shop that did the brakes can fix you up.

 

Do you think there's anything wrong with having the gap? Is it worth fixing?

 

I dont think that is it. I bet they left of the anti rattle clip or clips.

The pads are on with the pins...so I dont think they are going back and forth.
I think its side to side.

Make the brake guys look at it

 

Could be a u-joint .....

 

Maybe I sound naive when i ask this but can i inspect for those pins without removing the wheels?

 

Here is what ya do....drive up to the brake place....walk in....and say..."my brakes are clunking...could you please fix them?"

Dont be shy...if it started right after the work was done....it is something they did.

Parts dont wait for each other to break all at once. Most of the time...something new that spring up after a repair...is something you did, not a malicious part biding its time to double whammy you.