Honda-SOHC

I did indeed find some worn housing (inside) - and actually found one of the missing screws sitting in a lump of grease where I'd guess a previous owner put it for safe keeping (then forgot to put back in).  So hopefully that's the cause of the noise - but of course I wont know for certain until I get her back together again (and at present progress that could be some months away).  Never mind - I bought her as a project bike to gently ease me into working on bikes again - and I'm loving it some 30 odd years after I last had bikes - but this time I have a dry garage to work in and no time limit - not a wet cold pavement and needing to fix it to get to work the next day!  Luxury!

I have a 1976 400-4 (that'd currently in a state of clean-up (re-furb is to generous a term for what I'm doing!).  But - before starting work on her I found that in some gears (not always the same gear - no consistency) I could hear a strange rumble/scraping sound from the gear box / chain area.  At higher revs (ie. 5k and up) it goes.  Also - if, when I hear hear the noise, I quickly shut off power and re-apply it - the noise sometime goes. It's very odd and I cant for the life of me figure what could be causing it.  The clutch doesnt slip (indeed have had that out and checked it over and it seems fine (one plate - the inside - going out of tolerance - but I have swapped them around to be fair  .  I did find that the 2 bolts that hold the plate on the front chain sprocket are missing so wondered whether that plate has been moving about and making strange noises) - but this sound is noting I've heard on bikes before - indeed didnt hear when I first had the bike (have had to repair the rear tyre - could I have not put the wheel back on properly?!).  Any ideas / thoughts gratefully appreciated as I'm at a loss.  Thanks all.