Exhaust leak? Get yourself a scrap of hose, heater hose, fuel hose, etc, and probe around. If you can't find it, stick the hose down into the breather / filler hole on the valve covers
Could be many things. Try to start slow with the basics before some of us have you building a stroker to put in it.....lol. :D
Seriously though, did you run it hard just before this happened? Springs, rockers, pushrods.
Also, is it running smoothly and just making the noise? Or is there a detectable miss as it's making the noise?
Main reason I didn't go right to a exhaust leak first, was that it's doing it hot, and sounded intermittent thru my headphones. Many of them i've seen were much worse cold, and actually quieted down as things warmed up.
