Bike Doctor HELP!

the cassette has a knock or a rattle, a bit like chain out of alignment, but it isn't, and it only occurs in the middle of the cassette in rings 6&7 - could it be the cassette itself needs tightening (as the first 3 rings are normally a single piece?) or more sinister - like the free hub?

suggestions?

Well,

On which bike is it? How many miles on the bike/casette/chain combination?

-is the hanger of the rear derailer still in line?
-Aligning your rear derailer is an "art"in itself but it can be done by yourself. (in case you change the tension on the cable, you'd better shift up and down completely once or twice and then come back to the position before you make your judgement, this to prevent "cable sticking and unsticking")
-What about the right spacers inside the cassette, are these the right ones?
-What about the chain connection pin? Is it inserted in the correct way?
-You could check the fastening of the cassette locking nut. It should be tightened until it slips, after this there should be no lateral play on the cassette vs the freehub.

If non of this works, pls see a prof doctor and or Shedheadrabia

cheers,

Martin
It chorus, so about 1000m but it's not wear or alignment. Dropped the chain off and wheel out last night and there it rotation play between the rings of the cassette.

Ran out of time to fix, but a strip out and reassemble and tighten should sort it - I hope

Simple fix. Just the cassette to tighten. Flying machine No. 2 Can have a week off now

I assume the cassette loosens automatically with your speed?

I. Think you're right about loosening. The cassette has been used to Rev speeds, and now in the UK it only does that downhill, so on my routes which are mainly circular with more up than downhill the cassette is over rotating from it's use with you guys and can't adjust to my slow UK outings