so my car is making this clicking or tapping noise whenever I lift my foot off the gas padel, the frequency of the taps increases when on higher speeds. But once I accelerate, the sound disappears. I tried diagnosing it in a local workshop and they told me that it is the gearbox, however, I'm not sure about their inspection. Any help would be appreciated. I recently changed the car engine, after changing it the car wouldn't reverse due to the old transmission fluid, so I changed it and it was perfect afterwards until now.

Pay for it to be dealer inspected, then find out where you might want it to be serviced. At least you'll be 100% sure what is wrong.

Your description of the noises leads me to believe your engine is experiencing "knocking." You may have an issue with the variable valve timing (VVTi), the ignition timing, or the knock sensors. The sensors and controls or these systems need to be checked.

Sometimes a lack of regular oil changes can gum-up the VVTi. An oil additive like Seafoam or Marvel Mystery Oil can help "un-gum" the works over time. Make sure too that you are running a high-grade synthetic oil.