"Thunk" sound after caliper and brake pad replacement

[Hamster]

I'm posting this to make sure I'm not crazy or something. In June, I started getting poor gas mileage (32 mpg highway, when it should be around 38 mpg), shaking steering wheel when accelerating on highway, loud groaning noise from the right front wheel while holding down the brakes for more than a few seconds (like at a red light), a loud groan when releasing the brakes, as well as a ticking noise after a long drive. Long story short, the right front caliper was sticking due to rust. Basically, I was driving around with the pads constantly pressed against the rotor. Took the Yaris to a mechanic for the first time ever, and he replaced both front rotors, right side brake pads, right side brake hose, replaced the brake fluid, and did a general service and inspection of the other brakes. Car has only about 34,000 miles on the odometer, so the rusting was caused by time, not by wear-and-tear.

Anyway, I'm back to getting normal gas mileage, no ticking, steering wheel no longer shakes, and no groaning while stopped at a red light. Everything is working super. However, I can hear a mechanical "thunk" sound from the passenger side when I quickly release the brake pedal after holding it down for a while, such as after a light turns from red to green. I don't get a thunk when I very slowly taking my foot off the brake. It thunks when I suddenly release the brake pedal. This sound is not from the brake pedal itself, it's something mechanical. Are your guys' Yarises completely silent when releasing the brakes, such as after a light turns from red to green? Or does your car also make some sort of sound when you let go of the brakes? To tell the truth, I can't remember if my car always made a sound! Even though I've driven this car since new, I never paid attention. I want to check if it's normal to hear a slight thunk from somewhere on the passenger side after immediately taking your foot off the brake pedal. I can't figure out if the sound is coming from the new caliper, or if it's coming from under the hood. Don't know if I became hypersensitive to any sound the car makes, or if the mechanic didn't grease or screw on something properly. Want to know if I need to take the car back to the mechanic, or if everything's fine and I'm just a little jumpy. Thanks in advance for your replies!