Troublshooting noisy smps
Just maybes:

If any of these prove useful I'll come back and tidy up the answer. Otherwise no great value.

You've got a lovely pickup loop across R16 - why are FB R's across other side from both Vout and FB pin? |

C22 is in middle of that loop - noise filtered by C22 MAY be inductively coupled into loop.
Install C33 (small).
Temporarily move C22 physically and tack on in next best locn away from FB loop.

IP and OP caps could cause problems if well off values shown.

Data sheet does NOT say so but possible it is unstable if output cap ESR does not fall in a certain range. Varies with load. Check. 

What sort on in &amp; out caps? Ceramic? 

C33 = transient response speedup. Diagm says DNP. Is it populated? Try it.
What happens with small C33? 

What is load.
How does op waveform and amplitude change with load?