nevermind,
it was the air pump re-engaging after the revs drop, the front of the air pump stops for some reason, then when u come off the revs it clicks and re-starts.
is it meant to do that??
Yes,
The air pump only runs below 3000rpm to keep emissions low.
The thing on the front that you see engaging and disengaging is the electric clutch for the air pump.
Regards
Carl