Just fitted new chain and cassette and get a griding noise under load. Chain not slipping under load and shifting is fine. Can see the chain moving side to side on it’s way to the jockey wheels apparently co-inciding with the noise. Can also feel light vibration through the pedals on smooth road under load too.

If I put the bike on the workstand and turn the pedals nothing seems amiss.

Have replaced chain (SRAM for SRAM) and cassette (SLX instead of SRAM). Kept x9 mech jockey wheels and the SRAM chainset.
Changed the chain as it’s stretched by the 1%, fresh chain skipped on the old cassette hence new cassette. The new chain is only the third the bike has seen and the chain rings and jockey wheels don’t look worn.

Is it as simple as wait for things to bed-in?

Hard to tell. To me-

Possibility 1, chainrings are worn so the lower non-tensioned part of the chain isn’t releasing freely from the chainrings. Basically minor chainsuck, as I have just christened it. From this point, either the chainrings tooth pitch beds into the new chain pitch or, when the chain gets dry of lube and/or dirty, you then get full blown chain suck.

Possibility 2, you need a narrower chain e.g. you have used an eight speed chain.

Could be several things. First to check might be that you fed the chain properly through the derailleur cage. There’s be a little bridge between the two jockey wheels and it’s not uncommon for people to feed the chain down the wrong side of it.

Threefish has indeed come up with a possibility three. Done that myself 🙄

Thanks for thoughts: chain is definitly SRAM 9spd – at ‘970’ with 9spd on the magic link.
I did wonder about ‘minor chainsuck’ theory would be under load the chain is ‘stretched’ onto the top of the chainring, as the chain tries to depart at the bottom the chain has shrunk onto the chainring and is reluctant to leave.

Solution 1: change chainring (trying to find Truvativ spares)
Solution 2: put up with it and go heavy on the lube.

I second that felt a right idiot when I realised. Check quick though as they wear through ultra quick!

Just chucked lots of chain lube on the chain and compared the issue across the 3 chainrings. Issue definitly worse in the middle ring, minimal probs on other two. I think issue has eased with riding but it’s still there- oh well guess Bikefest on saturday will kill or cure it!

That confirms worn chainrings, as the middle wears most. You are more open to chainsuck at any point now. If it were my bike I’d replace the middle chainring, at least. I’ve always imagined old chainrings wear out chains and cassettes – although I’ve never carried out a scientific analysis 🙂