It is actually very good usually. During cruise when the autothrust is on and in MACH mode it is what is called a soft mode (I think that's the terminology) , it makes smaller changes in thrust and the deviations in speed are sometimes realatively large whilst the thrust progressively increases or decreases smoothly to counter the trend. The further away from the selected or managed bug the more aggressively it tries to catch it but it still attempts to do this smoothly.
If it is having trouble staying away from green dot or MMO I have come across a few guys who will select a speed (push the button to take it out of Mach and into Speed mode). The result of this is the A/THR is now very much tighter. Any deviation from this now selected SPEED results in an almost instantaneous correction from the trust to reverse the trend back to the selected value. It does not sound nice in the cabin although it is actually maintaining the target speed with much better accuracy. If you hear it doing this it should not concern you, it is in fact a sign the guys/girls are awake and very closely monitoring the situation.
If you heard rather spaced changes from near idle to climb then it is likely they had it in the soft mode with Mach selected (managed or selected but in Mach), this is just how it deals with large fluctuating winds as the speed can get quite far from the target value before it really goes after it.
If it was very quick with power on and then off, multiple times a minute then it was probably as I stated about them changing from MACH to SPEED.
Without going into the books I can't go into much more detail although am sure someone with far better knowledge than me will be along shortly.