The first thing to check is the crankshaft pulley they are known to come lose and make a clattering noise, but by your description it sounds like big end bearings. This is what happened to me last year, when starting the engine it would make a clattering noise and then disappear, if I increased the revs to about two thousand it would come back.Stripped the engine out to find one of the big end shells had spun.Sent the crankshaft away to be reground and tested unfortunately the crank was cracked which often happens when a shell has spun or the engine has seized. Spent the next few months looking for a recondition crankshaft at a reasonable price, plenty in the US but none in the UK, ended up buying a second engine and using the parts from both to get back on the road. Pic of bearing shell below.
