That is normal in most baitcasters, especially when the spool is fully loaded with line. Depending on the design of the system, some reels will do it more than others. What’s happening is that the pinon gear is slipping in the spool cross pin. When you crank the handle very quickly then stop the handle, the pinion gear stops moving in relation to the spool. This causes the pinon gear to bounce off the spool pin. The yoke springs are trying to force the pinion gear to reengage to the spool. But the spool is spinning too quickly for that to happen, so the pinion gear bounces off again. This results in the buzzing noise.
While this may seem like something is wrong with the reel, it is actually good that it happens. When you spin a loaded spool quickly, there is decent amount of energy being generated. If the pinion gear stayed fully engaged when you stop the handle, you would be putting a lot of strain on the gearing. The chance of something breaking would greatly increase. That being said, if you experience that slip during normal fishing conditions or for an extended amount of time, then you could possibly have a problem with pinion engagement to the spool.