most likely it is "normal", nothing to worry about and will not do any damage
like Ford-a-Tude said, it could be other things too -- but it sounds like you are describing "drive train lash" -- it is basically accumulated play from all the joints in the drive shaft and rear-end, and when you reverse the load on the driveshaft all that play comes out in one direction and then clunks in the other direction when you load it the other way again . . . if you get the technique down with the gas and clutch, you can avoid the loading/unloading and the clunking.
Here's what I think is happenning:
1) you are driving forward, the engine is loading the driveshaft "forward", all the play in the joints is taken out in the forward direction
2) when you go to shift, you let off on the gas just before you push in the clutch . . . so now the engine is pushing backwards on the driveshaft, and the play gets taken out in the "backwards" direction -- clunk! (most noticeable in low gears because that's where the engine braking is the strongest)
3) then you give it some gas and let out the clutch, the engine starts pushing on the driveshaft in the forward direction again, the play gets taken out in the forward direction . . . . clunk!
So, if this is really what is happening -- if you push the clutch in just a little sooner, before the engine braking can take the slack out backwards, you eliminate both clunks.
If it is really bad it probably means something is really worn out somewhere in the drivshaft so there is a lot of play and a lot of clunking.
| 2010 GT coupe | Daily Driver with some Steeda and GT500 take-off stuff for the occasional track day