Yeah. Just got my OB 6 and whenever I power it on I get the pop and some kind of whirring sound, almost like what a tape deck sounds like. Assuming this too is normal, though My Prophet 6 is nearly silent on power up/down.
To those saying you should turn off speakers when powering up, yes, obviously, but that doesn't mean this behavior isn't unusual. I have a Jx 8P, Juno 106, Moog Sub Phatty and Prophet 6 and none of them behave this way -- only the OB6, so clearly there's something about it specifically.