I get that sort of 'turbo-ish' spool up noise but it is coming from the cam gears. use to worry something was wrong, but found no wear or metal fines. probably the sound caused by the straight cut gear teeth and the back lash compensators.

05 speedmaster - 1100cc, 11:1 racing pistons, Carillo rods, thunderbike cams, ported and polished head, 2mm over intake and exhaust valves, Barnett kevlar clutch, scepter pipes, oversize manifolds, 45mm HSR's, TTP stage 4 firestarter