Engine - rattling noise

[Eas85]

So, on my way home from work today began to hear a little rattling noise whilst waiting for the lights to change. Got the bike home and put it on the stand, had a closer look and from hearing alone its coming from the top of the engine just below the petrol tank.
For now it just sounds like something has come loose and is being rattled by the movement of the engine.
I can't see anything but and I'm currently waiting for a the bike to cool down before I have a poke around, but does anyone have any ideas after that brief explanation?
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Currently riding a Honda Deauville NT650

[.....]

Possibly an engine bolt? Could be lots of things, I'd run the engine so the rattling happens while having a poke around and take it from there.

[Eas85]

on closer inspection it actually sounds like its coming from the exhaust, but nothing seems loose, and it sounds like it could be in the exhaust, but bike hasnt been dropped or knocked or anything
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Currently riding a Honda Deauville NT650

[Dr. Quack]

As the engine gets warmer and the oil gets thinner valve clearances can sound more prominent, it could also be the cam chain.

[Blau Zedong]

[Dr. Quack]

[Noxious89123]

That quote is wrong, its an OHV design, not OHC...
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'06 Honda CBR125RW-6 ~ '00 Honda CBR600F-Y ~ '07 Honda CBR600RR-7 ~ Bikeless
'53 Ford Ka 1.3 ~ '03 Vauxhall Astra SRi 1.8 ~ '52 Vauxhall Astra SRi 2.2 ~ '53 Vauxhall Astra GSi