I reply here before actually seeing vacuumdevil's video. Perhaps what I am about to say is in the video, making this redundant. If so, I'm sorry for wasting your time. If not, this might add some useful info to supplement my earlier reply.
If you take the tech drive assembly out of your Kirby, you will see that there is an axle onto which the two rear wheels attach. This axle can be removed if you take off the tech drive D/N lever (1 screw underneath the transmission) followed by the two axle mounts (1 on each side, each attached with 1 screw).
On this axle, you will see a number of elements. Notice that there is one large gear (blue arrow) and a piece just beside it (red arrow) with a "castle" shape. The large gear is always driven by the transmission and never disengages from it. You will find that this piece is free to spin on the axle. However, the element with the "castle" shape next to it is constrained to the axle such that it cannot rotate with respect to the axle but CAN slide along it. This piece slides up to the large gear when the vacuum is set to "D" and its "castle" pattern mates up with a complementary pattern on the large gear. Therefore, when the two elements are locked together, the vacuum can self-propel. Pressing the vacuum into "N" makes the "castle" piece slide away, and the two pieces disconnect, allowing the vacuum to freewheel.
The clicking noise occurs if the two pieces I discussed above do not fully separate when the vacuum is set to "N." One common cause of this is a worn cam lever, which you replaced. The other cause is a messed up D/N pedal. If replacing the cam lever doesn't solve the problem, then replacing the D/N pedal likely will.
However, even if you continue to use the vacuum with the clicking sound, you will not will not damage the transmission. Rather, only the two aforementioned (and separately replaceable) pieces could slowly wear down.
Good luck.
https://www.ebay.com/itm/Kirby-Rear...Gear-Assy-G3-thru-Avalir-555211-/183243131055
