Same as others on here my thoughts on it is 'clutch basket' or plates rattling in the basket. start the bike up on centre stand and hang over bike and listen close to the clutch case and pull the clutch lever right up against the bars and see if the noise diminishes, if it does then it is the clutch.
Careful running bike with oil filler cap off you might make a mess on the cylinder fins that will stink the garage out as it burns off.
A lot of FZS 600 Fazers have rattles from the clutch and sometimes just recording it makes it sound worse on the recorded file that it actually is.
My Mechanic replaced the whole of the clutch assembly on his Boxeye because it rattled and because he could, it was silent when he started it and about a month later he noticed it had a slight rattle back and within a few months it was as bad as it was before replacement.
My Foxeye has a slight rattle (Mine decreases by about 50% with the clutch lever pulled in to the bars) and I spoke to Darron about it and his advice is nothing to worry about, that was 3 years ago and to be honest I don't think it's any louder now than when I first noticed it.
If the bike is in a shed or garage or up against the side of your house when you are listening to it running the building will act like a resonator, bit like a guitar body amplifies the sound of the strings, the building will caused the sounds to bounce around and amp it up.
If it throws a con-rod through the front of the crankcase your next ride out I have never heard of you.
What I guess i'm saying for your own peace of mind get a professionals opinion, 25 quid for the professional opinion is much cheaper than a new lump for your bike.