What is the cause of a knocking noise around the gearbox area in chevrolet AVeo 2008

run the fault codes first to determine if the problem is electronic ( servos, solenoids , wiring and connections or TCM
it could be a coincidence about the noise in the transmission area so leave the new transmission out of the equation until all the possibilities are covered
has the transmission oil been change lately
drop the pan of the transmission and check for foreign material in the pan and filter
if you have something major , there will be evidence in that area

The first thing you need to check is for water contamination in the gearbox oil, if none try filling it with the correct procedure and reset adaptations, if there is water in the gearbox do not bother repairing it at a huge cost, they seldom get it right. Rather look for a good secondhand box that is the same make, the most common is the ZF6HP19

Firstly check if you have gearbox oil in it seldomly so happens that a gearbox makes a noise but when it does make a kind of winding noise the gear inside the box has gotten damaged. Sometimes when lucky if you add gearbox oil the noise goes away.

The noise will be in the brake disc rotors and could be the inside face cracked . Have a brake specialist shop do the check as the knocking noise under brakes will have nothing to do with a transmission.

knocking noise could be worn drive constant velocity joints.check cv joint boots for tear leaking out the lubricant.could also have worn side gear shaft counterbore in the differential case.

A gearbox will not "knock" in the usual sense, that is said of an engine running on petrol which is a lower octane than it needs. The sound is more of a light rattle or pinging.

If your manual gearbox has had the wrong oil in it for only a week or 2 and has not been thrashed, it is unlikely it has been damaged at all. Just drain and refill with the right grade.

If an auto gearbox, using the wrong oil can cause damage, as it may be far too thick. Drain and refill 2-3 times with the correct grade, running the engine until warm, but not driving around, in between. Then go for a drive without thrashing the car, and see if you can notice anything abnormal.

If you can, take it to an auto trans shop for advice.
Then

most likely you have a wheel sensor that has come loose or another item that has gottgen in around the sensor and causing it to read faulty. check the wheel where your hearing the noise and most likly you will find a loose sensor or other debris.

you dont ---end of story it should never need changing on a modern vehicle ,unless you take the drive shafts out to do a clutch then you top it up through a filler plug at halfway level on the gearbox ,Dont make work .

IF a transmission (not gear box) gets low on transmission fluid - that can certainly cause bad damage..
A automatic transmission rund by hydrolic bands - not gears..

A re-built trani is pretty much as good as a new one - and certainly less expensive than a new one..

Most all auto trani's are re-built rather than being replaced with a new one..

this easy, drain oil from gear box remove that drive shaft (u need a ball joint splitter)
replace gearbox seal, pry out old 1 with screw driver, gently tap new in, oil edge of new seal 1st. replace oil, ps dont leave to long as will damage gearbox when lack of oil