I know things... A lot of things.
Join Date: Feb 2015
Motorcycle: GSXR 750 K4
Re: At key on and during fuel pump charge, clicking?
It's likely that you're hearing the STVAs cycling. During each startup the ECU calibrates itself by turning the plates fully open until they bang against the upper stop for half a second or so. After that the ECU knows that when the STPS reads such and such the STVs are fully open, so it doesn't keep try to open the STVs past fully open every time you go WOT, simply because it's slightly miscalibrated.
That's my understanding at least; I haven't been able to confirm that from an official source, such as the service manual. Still, every GSXR I've seen up close or on YouTube seems to do it and, if I remember correctly, others here have made the same hypothesis. Anyway, when the plates bang against the upper stops, it makes a rapid clicking sound. Your sound might be something else of course. Try listening to videos on YouTube, to see if other bikes make the same sound, or record one of yours and upload it for us to listen.