Strange noise from engine in low revs and various gears

[DayvW]

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.

[ka-ja]

Hi,
    First thing to check (after a good clean) is the castings inner and outer of the drive sprocket, if the securing washer has turned, the sprocket could be floating sideways, and the chain ends up chewing alloy, if that's all OK then re-assess---Ken

[DayvW]

Hi Ken,
Thanks for your response - I shall take a good look later as you suggest. As a further clue - the noise does sound a bit like it's not properly in gear (but it clearly is as it pulls just fine and revs dont suddenly sky rocket).  But - I'll take a look at those housings first and report back.  Thanks again for taking the time.

[DayvW]

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!

[ka-ja]

Hi,
     I know just how things seem to slip away with time, My cafe racer was supposed to be finished for Barton Bike Night last year, I could end up struggling for this year, there just isn't enough hours in the day, even if you do nothing!
    Don't forget to check condition of the "splined" sprocket holding washer, and fit the two holding screws with a spot of nut-lock, after that, if the noise persists it may not be good news, you could be looking at the drive end bearing or even bent selecters, best of luck----Ken

[DayvW]

I'll be sure to make sure the screws are not going anywhere - thanks Ken.  And Oddjob - I did wonder about wheel alignment (and I think the chain is standard  -but I'll take a closer look).  I thought the alignment was OK but I think the noise only started after I re-fitted the wheel following a puncture repair (certainly wasnt there when I first bought the bike in June - so hoping it's that / the missing screws and nothing more serious (I rode her gently so cant believe that even I can have damaged her in such a short time )  Time will tell.)  Thank you both for your inputs - and once I have more I will update this thread.