Where is this sound coming from?

Hey guys, for the past 2 months I've been plagued by this sound. I've checked the spindle, cranks, regreased and retightened them. I know it's not the seatpost or saddle, because it happens when I'm standing and pedalling. Sometimes it disappears, but usually it comes back. Bike is Sunday hi-c. Looking for pointers on what else to check, thanks.