It's like something loose with a tinkling sound at 30 to 40 on/off throttle. Normally if really loose it rattles like someone tapping a spanner on the exhaust at tickover when cold.
As a bit more detail I’ve just taken the callipers off and it’s the same on both of them.The knock is because the pad is held in with one pin so the pad is free to rotate around it and the knock is coming from the the pad rotating into the calliper.So I’m assuming this in normal even though it’s quite a knock?
I know that you have probably already checked this, but have you made sure the the spring washers on the disc bobbins aren't tired, and the noise is the disc jiggling on the carrier. I had a ZZR600 that did exactly what you describe, and it was the spring washers on the bobbins which had just lost their "Spring".